I've had this issue for a while and even the latest version saying "hopefully fixed byg incompatibity crash" hasn't resolved the issue. It happens when loading some specific chucks in the overworld and nothing I've tried had resolved this issue.
here are my logs:

Time: 9/10/21 8:05 PM
Description: Exception ticking world

java.util.ConcurrentModificationException: null
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_51] {}
at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_51] {}
at com.google.common.collect.Iterators$3.next(Iterators.java:174) ~[guava-21.0.jar:?] {}
at net.minecraft.world.server.ChunkManager.lambda$null$25(ChunkManager.java:761) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,xf:optiforge:default}
at net.minecraft.world.server.ChunkManager$$Lambda$49456/1098754553.apply(Unknown Source) ~[?:?] {}
at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[datafixerupper-4.0.26.jar:?] {re:classloading}
at com.mojang.datafixers.util.Either$$Lambda$49281/1395265571.apply(Unknown Source) ~[?:?] {}
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[datafixerupper-4.0.26.jar:?] {}
at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[datafixerupper-4.0.26.jar:?] {re:classloading}
at net.minecraft.world.server.ChunkManager.lambda$func_219200_b$26(ChunkManager.java:733) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,xf:optiforge:default}
at net.minecraft.world.server.ChunkManager$$Lambda$49453/1980819461.apply(Unknown Source) ~[?:?] {}
at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442) ~[?:1.8.0_51] {}
at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter.func_219083_b(SourceFile:58) ~[?:?] {re:classloading}
at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter$$Lambda$49077/2046562206.run(Unknown Source) ~[?:?] {}
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(ThreadTaskExecutor.java:189) ~[?:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.func_213166_h(ServerChunkProvider.java:519) ~[?:?] {re:classloading}
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(ThreadTaskExecutor.java:151) ~[?:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.func_213168_p(ServerChunkProvider.java:527) ~[?:?] {re:classloading}
at net.minecraft.world.server.ServerChunkProvider.func_217234_d(ServerChunkProvider.java:272) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.func_213205_aW(MinecraftServer.java:722) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_213168_p(MinecraftServer.java:711) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213160_bf(ThreadTaskExecutor.java:128) ~[?:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.server.MinecraftServer.func_213202_o(MinecraftServer.java:696) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:646) [?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:232) [?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer$$Lambda$48733/1407587508.run(Unknown Source) [?:?] {}
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51] {}

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

-- Head --
Thread: Server thread
Stacktrace:
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_51] {}
at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_51] {}
at com.google.common.collect.Iterators$3.next(Iterators.java:174) ~[guava-21.0.jar:?] {}
at net.minecraft.world.server.ChunkManager.lambda$null$25(ChunkManager.java:761) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,xf:optiforge:default}
at net.minecraft.world.server.ChunkManager$$Lambda$49456/1098754553.apply(Unknown Source) ~[?:?] {}
at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[datafixerupper-4.0.26.jar:?] {re:classloading}
at com.mojang.datafixers.util.Either$$Lambda$49281/1395265571.apply(Unknown Source) ~[?:?] {}
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[datafixerupper-4.0.26.jar:?] {}
at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[datafixerupper-4.0.26.jar:?] {re:classloading}
at net.minecraft.world.server.ChunkManager.lambda$func_219200_b$26(ChunkManager.java:733) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,xf:optiforge:default}
at net.minecraft.world.server.ChunkManager$$Lambda$49453/1980819461.apply(Unknown Source) ~[?:?] {}
at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:577) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442) ~[?:1.8.0_51] {}
at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter.func_219083_b(SourceFile:58) ~[?:?] {re:classloading}
at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter$$Lambda$49077/2046562206.run(Unknown Source) ~[?:?] {}
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(ThreadTaskExecutor.java:189) ~[?:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.func_213166_h(ServerChunkProvider.java:519) ~[?:?] {re:classloading}
at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(ThreadTaskExecutor.java:151) ~[?:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,xf:OptiFine:default,re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.func_213168_p(ServerChunkProvider.java:527) ~[?:?] {re:classloading}
at net.minecraft.world.server.ServerChunkProvider.func_217234_d(ServerChunkProvider.java:272) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.func_213205_aW(MinecraftServer.java:722) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
-- Affected level --
Details:
All players: 1 total; [ServerPlayerEntity['Mizokuu'/191, l='ServerLevel[Valemist]', x=-2448.62, y=110.22, z=1940.07]]
Chunk stats: ServerChunkCache: 3416
Level dimension: minecraft:overworld
Level spawn location: World: (0,72,320), Chunk: (at 0,4,0 in 0,20; contains blocks 0,0,320 to 15,255,335), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 5233181 game time, 43202 day time
Level name: Valemist
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Level weather: Rain time: 73552 (now: false), thunder time: 129585 (now: false)
Known server brands: forge
Level was modded: true
Level storage version: 0x04ABD - Anvil
Stacktrace:
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:854) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:787) ~[?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:118) ~[?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:642) [?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:232) [?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:betterendforge.mixins.json:MinecraftServerMixin,pl:mixin:APP:structure_gel.mixins.json:MinecraftServerMixin,pl:mixin:APP:byg.mixins.json:server.MixinMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer$$Lambda$48733/1407587508.run(Unknown Source) [?:?] {}
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51] {}