My world start up crashed twice and when I checked the report it seemed there was perhaps an issue with Enhanced Celestials. I removed the mod and the world started up alright.

---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 2/15/21 9:33 PM
Description: Unexpected error

java.lang.IllegalStateException: Lock is no longer valid
at net.minecraft.world.storage.SaveFormat$LevelSave.func_237301_i_(SaveFormat.java:296) ~[?:?] {re:mixin,re:computing_frames,re:classloading}
at net.minecraft.world.storage.SaveFormat$LevelSave.func_237298_f_(SaveFormat.java:360) ~[?:?] {re:mixin,re:computing_frames,re:classloading}
at net.minecraft.server.MinecraftServer.func_184109_z(MinecraftServer.java:769) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:enhancedcelestials.mixins.json:MixinMinecraftServer,pl:mixin:APP:charm.mixins.json:accessor.MinecraftServerAccessor,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_184106_y(MinecraftServer.java:763) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:enhancedcelestials.mixins.json:MixinMinecraftServer,pl:mixin:APP:charm.mixins.json:accessor.MinecraftServerAccessor,pl:mixin:A}
at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:699) ~[?:?] {re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:976) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) [forge-1.16.4-35.1.37.jar:35.1] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$485/1788432625.call(Unknown Source) [forge-1.16.4-35.1.37.jar:35.1] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?] {}

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Render thread
Stacktrace:
at net.minecraft.world.storage.SaveFormat$LevelSave.func_237301_i_(SaveFormat.java:296) ~[?:?] {re:mixin,re:computing_frames,re:classloading}
at net.minecraft.world.storage.SaveFormat$LevelSave.func_237298_f_(SaveFormat.java:360) ~[?:?] {re:mixin,re:computing_frames,re:classloading}
at net.minecraft.server.MinecraftServer.func_184109_z(MinecraftServer.java:769) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:enhancedcelestials.mixins.json:MixinMinecraftServer,pl:mixin:APP:charm.mixins.json:accessor.MinecraftServerAccessor,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_184106_y(MinecraftServer.java:763) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:enhancedcelestials.mixins.json:MixinMinecraftServer,pl:mixin:APP:charm.mixins.json:accessor.MinecraftServerAccessor,pl:mixin:A}
-- Affected level --
Details:
All players: 1 total; [ClientPlayerEntity['Azteca09'/217, l='ClientLevel', x=8.50, y=65.00, z=8.50]]
Chunk stats: Client Chunk Cache: 441, 0
Level dimension: minecraft:overworld
Level spawn location: World: (8,64,8), Chunk: (at 8,4,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 0 game time, 0 day time
Server brand: NULL
Server type: Integrated singleplayer server
Stacktrace:
at net.minecraft.client.world.ClientWorld.func_72914_a(ClientWorld.java:617) ~[?:?] {re:mixin,re:classloading,xf:OptiFine:default,pl:mixin:APP:abnormals_core.mixins.json:client.ClientWorldMixin,pl:mixin:A}
at net.minecraft.client.Minecraft.func_71396_d(Minecraft.java:2024) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:628) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) [forge-1.16.4-35.1.37.jar:35.1] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$485/1788432625.call(Unknown Source) [forge-1.16.4-35.1.37.jar:35.1] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?] {}

-- System Details --
Details:
Minecraft Version: 1.16.4
Minecraft Version ID: 1.16.4
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 490216864 bytes (467 MB) / 2147483648 bytes (2048 MB) up to 2147483648 bytes (2048 MB)
CPUs: 4
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
ModLauncher: 8.0.9+86+master.3cf110c
ModLauncher launch target: fmlclient
ModLauncher naming: srg
ModLauncher services:
/mixin-0.8.2.jar mixin PLUGINSERVICE
/eventbus-4.0.0.jar eventbus PLUGINSERVICE
/forge-1.16.4-35.1.37.jar object_holder_definalize PLUGINSERVICE
/forge-1.16.4-35.1.37.jar runtime_enum_extender PLUGINSERVICE
/accesstransformers-3.0.1.jar accesstransformer PLUGINSERVICE
/forge-1.16.4-35.1.37.jar capability_inject_definalize PLUGINSERVICE
/forge-1.16.4-35.1.37.jar runtimedistcleaner PLUGINSERVICE
/mixin-0.8.2.jar mixin TRANSFORMATIONSERVICE
/OptiFine_1.16.5_HD_U_G6.jar OptiFine TRANSFORMATIONSERVICE
/forge-1.16.4-35.1.37.jar fml TRANSFORMATIONSERVICE
FML: 35.1
Forge: net.minecraftforge:35.1.37
FML Language Providers:
[email protected]
minecraft@1
Mod List:
forge-1.16.4-35.1.37-client.jar |Minecraft |minecraft |1.16.4 |DONE |NOSIGNATURE
TenshiLib-1.16.3-1.3.0.jar |TenshiLib |tenshilib |1.16.3-1.3.0 |DONE |NOSIGNATURE
infernal-expansion-forge-1.3.jar |Infernal Expansion |infernalexp |Release 1.3 |DONE |NOSIGNATURE
EndRemastered-R352.jar |End Remastered 3.5.2 |endrem |1.16.3 |DONE |NOSIGNATURE
spiders-2.0-1.16.4-1.0.4 (1).jar |Spiders 2.0 |spiderstpo |1.0.4 |DONE |NOSIGNATURE
jei-1.16.4-7.6.1.71.jar |Just Enough Items |jei |7.6.1.71 |DONE |NOSIGNATURE
abnormals_core-1.16.4-3.0.6.jar |Abnormals Core |abnormals_core |3.0.6 |DONE |NOSIGNATURE
upgrade_aquatic-1.16.5-3.0.1.jar |Upgrade Aquatic |upgrade_aquatic |3.0.1 |DONE |NOSIGNATURE
majrusz-library-1.16.4-1.3.0.jar |Majrusz Library |majrusz_library |1.3.0 |DONE |NOSIGNATURE
strawgolem-1.16-1.9.jar |Straw Golem |strawgolem |1.16-1.9 |DONE |NOSIGNATURE
curios-forge-1.16.5-4.0.4.0.jar |Curios API |curios |1.16.5-4.0.4.0 |DONE |NOSIGNATURE
savageandravage-1.16.4-3.0.0.jar |Savage & Ravage |savageandravage |3.0.0 |DONE |NOSIGNATURE
create-mc1.16.3_v0.3e.jar |Create |create |mc1.16.3_v0.3e |DONE |NOSIGNATURE
BetterCaves-Forge-1.16.4-1.1.1.jar |YUNG's Better Caves |bettercaves |1.16.4-1.1.1 |DONE |NOSIGNATURE
Waystones_1.16.3-7.3.1.jar |Waystones |waystones |7.3.1 |DONE |NOSIGNATURE
PassableFoliage-1.16.4-2.2.0.jar |Passable Foliage |passablefoliage |2.2.0 |DONE |NOSIGNATURE
enchantedwolves-1.1.0.1.jar |Enchanted Wolves |enchantedwolves |1.1.0.1 |DONE |NOSIGNATURE
EpicFightMod-1.16.4-1.1.2.jar |Epic Fight Mod |epicfight |1.0.0 |DONE |NOSIGNATURE
citadel-1.6.1.jar |Citadel |citadel |1.6.1 |DONE |NOSIGNATURE
alexsmobs-1.5.1.jar |Alex's Mobs |alexsmobs |1.5.1 |DONE |NOSIGNATURE
AutoRegLib-1.6-47.jar |AutoRegLib |autoreglib |1.6-47 |DONE |NOSIGNATURE
Artifacts-1.16.5-2.7.7.jar |Artifacts |artifacts |1.16.5-2.7.7 |DONE |NOSIGNATURE
YungsApi-1.16.4-Forge-4.jar |YUNG's API |yungsapi |1.16.4-Forge-4 |DONE |NOSIGNATURE
OuterEnd-0.2.8.jar |Outer End |outer_end |0.2.4 |DONE |NOSIGNATURE
decorative_blocks-1.16.4-1.7.2.jar |Decorative Blocks |decorative_blocks |1.7.2 |DONE |NOSIGNATURE
Regrowth-1.16.4-1.16.4-1.35.0.18.jar |Regrowth Mod |regrowth |1.16.4-1.35.0.18 |DONE |NOSIGNATURE
guardvillagers-1.16.4-1.1.9.jar |Guard Villagers |guardvillagers |1.1.9 |DONE |NOSIGNATURE
endergetic-1.16.4-3.0.0 (1).jar |The Endergetic Expansion |endergetic |3.0.0 |DONE |NOSIGNATURE
majruszs-difficulty-1.16.4-0.4.1.jar |Majrusz's Progressive Difficul|majruszs_difficulty |0.4.1 |DONE |NOSIGNATURE
iceandfire-2.1.5-1.16.4.jar |Ice and Fire |iceandfire |2.1.5-1.16.4 |DONE |NOSIGNATURE
Hwyla-forge-1.10.11-B78_1.16.2.jar |Waila |waila |1.10.11-B78_1.16.2 |DONE |NOSIGNATURE
enhancedcelestials-1.0.1-1.16.4.jar |Enhanced Celestials |enhancedcelestials |1.0.1-1.16.4 |DONE |NOSIGNATURE
FpsReducer-mc1.16.4-1.18.jar |FPS Reducer |fpsreducer |mc1.16.4-1.18 |DONE |NOSIGNATURE
enchantwithmob-1.16.4-2.3.3.jar |Enchant With Mob |enchantwithmob |1.16.4-2.3.3 |DONE |NOSIGNATURE
forge-1.16.4-35.1.37-universal.jar |Forge |forge |35.1.37 |DONE |22:af:21:d8:19:82:7f:93:94:fe:2b:ac:b7:e4:41:57:68:39:87:b1:a7:5c:c6:44:f9:25:74:21:14:f5:0d:90
Quark-r2.4-304.jar |Quark |quark |r2.4-304 |DONE |NOSIGNATURE
charm-forge-1.16.4-2.2.2.jar |Charm |charm |2.2.2 |DONE |NOSIGNATURE
ImprovedMobs-1.16.3-1.7.3.jar |Improved Mobs Mod |improvedmobs |1.16.3-1.7.3 |DONE |NOSIGNATURE
supplementaries-1.16.5-0.9.24b.jar |Supplementaries |supplementaries |1.16.5-0.9.24b |DONE |NOSIGNATURE
SoundFilters-0.14_for_1.16.2.jar |Sound Filters |soundfilters |0.14_for_1.16.2 |DONE |NOSIGNATURE
infernalmobs-1.16.2.1.jar |Infernal Mobs |infernalmobs |1.16.2.1 |DONE |NOSIGNATURE
Crash Report UUID: 12f21dce-f673-43b7-b228-d702849a64e8
Launched Version: 1.16.4-forge-35.1.37
Backend library: LWJGL version 3.2.2 build 10
Backend API: Intel(R) HD Graphics 520 GL version 4.5.0 - Build 24.20.100.6293, Intel
GL Caps: Using framebuffer using OpenGL 3.0
Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'forge'
Type: Client (map_client.txt)
Graphics mode: fancy
Resource Packs: mod_resources, vanilla, quark:emote_resources (incompatible)
Current Language: English (US)
CPU: 4x Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
OptiFine Version: OptiFine_1.16.5_HD_U_G6
OptiFine Build: 20210117-013236
Render Distance Chunks: 8
Mipmaps: 4
Anisotropic Filtering: 1
Antialiasing: 0
Multitexture: false
Shaders: null
OpenGlVersion: 4.5.0 - Build 24.20.100.6293
OpenGlRenderer: Intel(R) HD Graphics 520
OpenGlVendor: Intel
CpuCount: 4

Try this: https://www.curseforge.com/minecraft/mc-mods/enhanced-celestials/files/3430183

Reopen this report if the issue persists