Fabric works fine until I try to play with any mod other than modlist, then it either crashes on startup or crashes when making world. There was the one time it worked, though I can't remember what mod I tried that with.

---- Minecraft Crash Report ----
// There are four lights!

Time: 3/12/20 1:49 PM
Description: Initializing game

java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors!
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:55)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:34)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:399)
at net.minecraft.client.main.Main.main(Main.java:166)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:192)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:138)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)
Suppressed: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:364)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:191)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178)
at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23)
at net.fabricmc.loader.launch.knot.KnotClassDelegate.loadClassData(KnotClassDelegate.java:174)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:143)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at paulevs.betternether.structures.StructureNBT.readStructureFromJar(StructureNBT.java:88)
at paulevs.betternether.structures.StructureNBT.(StructureNBT.java:31)
at paulevs.betternether.structures.bones.StructureBoneReef.(StructureBoneReef.java:13)
at paulevs.betternether.biomes.NetherBoneReef.(NetherBoneReef.java:12)
at paulevs.betternether.registers.BiomesRegister.(BiomesRegister.java:45)
at paulevs.betternether.BetterNether.onInitialize(BetterNether.java:30)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient$$Lambda$1444/869085647.accept(Unknown Source)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:48)
... 11 more
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [immersive_portals.mixins.json:MixinMinecraftServer] from phase [DEFAULT] in config [immersive_portals.mixins.json] FAILED during APPLY
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:643)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinApplyError(MixinProcessor.java:595)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:356)
... 25 more
Caused by: org.spongepowered.asm.mixin.injection.throwables.InvalidInjectionException: Critical injection failure: @Inject annotation on onFinishedLoadingAllWorlds could not find any targets matching 'Lnet/minecraft/server/MinecraftServer;method_3735(Ljava/lang/String;Ljava/lang/String;JLnet/minecraft/class_1942;Lcom/google/gson/JsonElement;)V' in net.minecraft.server.MinecraftServer. Using refmap immersive-portals-refmap.json [PREINJECT Applicator Phase -> immersive_portals.mixins.json:MixinMinecraftServer -> Prepare Injections -> -> handler$zfg000$onFinishedLoadingAllWorlds(Ljava/lang/String;Ljava/lang/String;JLnet/minecraft/class_1942;Lcom/google/gson/JsonElement;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V -> Parse]
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.findMethods(InjectionInfo.java:543)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.readAnnotation(InjectionInfo.java:280)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:267)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:259)
at org.spongepowered.asm.mixin.injection.struct.CallbackInjectionInfo.(CallbackInjectionInfo.java:46)
at sun.reflect.GeneratedConstructorAccessor15.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo$InjectorEntry.create(InjectionInfo.java:139)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.parse(InjectionInfo.java:595)
at org.spongepowered.asm.mixin.transformer.MixinTargetContext.prepareInjections(MixinTargetContext.java:1236)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.prepareInjections(MixinApplicatorStandard.java:1033)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.applyMixin(MixinApplicatorStandard.java:388)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:320)
at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:345)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:570)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:351)
... 25 more

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

-- Head --
Thread: Render thread
Stacktrace:
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:55)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:34)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointClient.start(EntrypointClient.java:32)
at net.minecraft.class_310.(class_310.java:399)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.main.Main.main(Main.java:166)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.fabricmc.loader.game.MinecraftGameProvider.launch(MinecraftGameProvider.java:192)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:138)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)

-- System Details --
Details:
Minecraft Version: 20w11a
Minecraft Version ID: 20w11a
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: 2100813696 bytes (2003 MB) / 2650800128 bytes (2528 MB) up to 5368709120 bytes (5120 MB)
CPUs: 4
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx5G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
Fabric Mods:
betternether: Better Nether 2.3.0
fabric: Fabric API 0.5.3+build.308-1.16
fabric-api-base: Fabric API Base 0.1.2+b7f9825d3d
fabric-biomes-v1: Fabric Biomes (v1) 0.2.1+b7d52fa53d
fabric-blockrenderlayer-v1: Fabric BlockRenderLayer Registration (v1) 1.1.4+c6a8ea893d
fabric-commands-v0: Fabric Commands (v0) 0.1.2+b7f9825d3d
fabric-containers-v0: Fabric Containers (v0) 0.1.6+0aae0f413d
fabric-content-registries-v0: Fabric Content Registries (v0) 0.1.3+b7f9825d3d
fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.1.2+b7f9825d3d
fabric-dimensions-v1: fabric-dimensions-v1 0.3.0+b17265713d
fabric-events-interaction-v0: Fabric Events Interaction (v0) 0.3.0+04b997943d
fabric-events-lifecycle-v0: Fabric Events Lifecycle (v0) 0.1.2+b7f9825d3d
fabric-item-groups-v0: Fabric Item Groups (v0) 0.1.9+5b8a535a3d
fabric-keybindings-v0: Fabric Key Bindings (v0) 0.1.1+dfdb52d63d
fabric-loot-tables-v1: Fabric Loot Tables (v1) 0.1.6+2f56dff23d
fabric-mining-levels-v0: Fabric Mining Levels (v0) 0.1.2+5b8a535a3d
fabric-models-v0: Fabric Models (v0) 0.1.0+dfdb52d63d
fabric-networking-blockentity-v0: Fabric Networking Block Entity (v0) 0.2.4+9421522d3d
fabric-networking-v0: Fabric Networking (v0) 0.1.8+9421522d3d
fabric-object-builders-v0: Fabric Object Builders (v0) 0.1.4+b7d52fa53d
fabric-particles-v1: fabric-particles-v1 0.1.1+dfdb52d63d
fabric-registry-sync-v0: Fabric Registry Sync (v0) 0.2.7+9421522d3d
fabric-renderer-api-v1: Fabric Renderer API (v1) 0.2.11+2f56dff23d
fabric-renderer-indigo: Fabric Renderer - Indigo 0.2.25+2f56dff23d
fabric-renderer-registries-v1: Fabric Renderer Registries (v1) 2.0.1+5a0f9a603d
fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.1.3+b7f9825d3d
fabric-rendering-fluids-v1: Fabric Rendering Fluids (v1) 0.1.6+12515ed93d
fabric-rendering-v0: Fabric Rendering (v0) 1.1.0+534104903d
fabric-rendering-v1: Fabric Rendering (v1) 0.1.0+534104903d
fabric-resource-loader-v0: Fabric Resource Loader (v0) 0.1.12+5b8a535a3d
fabric-tag-extensions-v0: Fabric Tag Extensions (v0) 0.1.3+abd915803d
fabric-textures-v0: Fabric Textures (v0) 1.0.4+821cdba73d
fabricloader: Fabric Loader 0.7.8+build.187
immersive_portals: Immersive Portals 0.2
minecraft: Minecraft 1.16-alpha.20.11.a
modmenu: Mod Menu 1.10.2+build.1
Launched Version: fabric-loader-0.7.8+build.187-20w11a
Backend library: LWJGL version 3.2.2 build 10
Backend API: NO CONTEXT
GL Caps:
Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fabric'
Type: Client (map_client.txt)
CPU:

This is an issue with Immersive Portals, not Fabric API.

19w10a broke many mods, and Immersive Portals is probably one of them. This issue affects 19w11a, too.

See if there is a newer version of Immersive Portals that you can use and report any issues you find with it to them.

Got it.
I'll close this, sorry.