That's quite a crash you've got there! The error java.util.ConcurrentModificationException usually points to a problem where the game is trying to modify a list of things (in this case, likely entities) at the same time it's trying to go through that list.
Looking at the stack trace, it seems to be happening within the entity management system (PersistentEntitySectionManager and ChunkMap). This could be caused by a few things, often related to mods interacting in unexpected ways.
Here's a breakdown of what might be going on and some steps you can take to troubleshoot:
Mod Conflicts: This is the most likely culprit given the number of mods you have installed. Some mods might be trying to manipulate entities or chunk data in a way that isn't thread-safe or compatible with other mods.
Lithium Mod: The crash report shows mixins from the lithium mod involved in entity management. While Lithium is generally an optimization mod, it can sometimes cause issues with other mods that heavily modify entity behavior or chunk loading.
ModernFix Mod: Similarly, modernfix is also listed in the mixins related to chunk loading and entity management. While it aims to fix issues, it could potentially have a conflict in your specific mod setup.
Entity Ticking Issues: Something might be causing entities to be added or removed from the world in a way that disrupts the game's internal processes.
Check Incompatible Mods: Your crash report explicitly flags several mods as "incompatible":
dungeon_crawl_dimensional_patch
You should remove all of these incompatible mods. They are very likely contributing to your issues.
Test with Essential Mods Only: After removing the incompatible mods, try running your server with only the core mods like Forge and any essential API mods that other mods depend on (like Geckolib, KumaAPI, etc.). If the server runs without crashing, you know the issue lies with one of the other mods.
Gradually Add Mods Back: If the server runs fine with the bare minimum, start adding your other mods back in small groups. After adding each group, restart the server and see if the crash reappears. This will help you pinpoint which mod or combination of mods is causing the conflict.
Look for Mod Updates: Once you identify a problematic mod (or mods), check if there are any updates available. Mod authors often release updates to fix bugs and improve compatibility with other mods and newer Minecraft versions.
Check Mod Configuration: Some mods have configuration files that allow you to adjust their behavior. It's worth looking at the configs of the mods involved in the crash (especially Lithium and ModernFix, and any mods that affect entities or chunks) to see if there are any settings that might be causing issues.
Consider Removing Optimization Mods Temporarily: While optimization mods like Lithium and Radium aim to improve performance, they sometimes achieve this by making significant changes to the game's internal workings, which can lead to incompatibilities. Try removing them temporarily to see if the crashes stop.
Given the sheer number of incompatible mods listed, I strongly recommend starting by removing all of them. That alone might resolve your issue. Let me know how it goes after that, and if you still encounter crashes, we can dive deeper into the remaining mods. Good luck!