Crashes while exploring the world

---- Minecraft Crash Report ----
// Hi. I'm Minecraft, and I'm a crashaholic.

Time: 2023-11-10 13:50:36
Description: Exception in server tick loop

java.util.ConcurrentModificationException: null
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1597) ~[?:?] {}
at java.util.HashMap$KeyIterator.next(HashMap.java:1620) ~[?:?] {}
at java.lang.Iterable.forEach(Iterable.java:74) ~[?:?] {re:mixin}
at net.minecraft.server.level.DistanceManager.m_140805_(DistanceManager.java:119) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B}
at net.minecraft.server.level.ServerChunkCache.m_8489_(ServerChunkCache.java:273) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:cupboard.mixins.json:ChunkLoadDebug,pl:mixin:APP:smoothchunk.mixins.json:ServerChunkCacheMixin,pl:mixin:APP:gtceu-common.mixins.json:ServerChunkProviderMixin,pl:mixin:APP:modernfix-common.mixins.json:bugfix.chunk_deadlock.ServerChunkCacheMixin,pl:mixin:APP:modernfix-forge.mixins.json:bugfix.chunk_deadlock.ServerChunkCache_CurrentLoadingMixin,pl:mixin:A}
at net.minecraft.server.level.ServerChunkCache$MainThreadExecutor.m_7245_(ServerChunkCache.java:545) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B}
at net.minecraft.server.level.ServerChunkCache.m_8466_(ServerChunkCache.java:269) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:cupboard.mixins.json:ChunkLoadDebug,pl:mixin:APP:smoothchunk.mixins.json:ServerChunkCacheMixin,pl:mixin:APP:gtceu-common.mixins.json:ServerChunkProviderMixin,pl:mixin:APP:modernfix-common.mixins.json:bugfix.chunk_deadlock.ServerChunkCacheMixin,pl:mixin:APP:modernfix-forge.mixins.json:bugfix.chunk_deadlock.ServerChunkCache_CurrentLoadingMixin,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_129961_(MinecraftServer.java:758) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_7245_(MinecraftServer.java:747) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:A}
at net.minecraft.util.thread.BlockableEventLoop.m_18701_(BlockableEventLoop.java:139) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.m_130012_(MinecraftServer.java:733) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_130011_(MinecraftServer.java:665) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_206580_(MinecraftServer.java:251) ~[client-1.20.1-20230612.114412-srg.jar%23896!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:A}
at java.lang.Thread.run(Thread.java:842) ~[?:?] {re:mixin}

Try disabling the Smooth Chunk Save mod. It looks like it is conflicting with Modernfix

#406 (comment)