Last night a user on my server reported a crashing issue. The crash would happen anytime that user attempted to log back in or anyone entered their relative area. I was able to confirm this myself and got stuck in a bit of a login loop doing so.
After looking through crash logs (See most recent crash attached) it appears to be something related to a Bee Nest and a function to replace entities (I assume something like replacing bees with Combees or something). I was able to remove the beehive by spamming the setblock command in the server console as I logged in and loaded the area. I haven't seen any crashes since, though it has only been a few minutes of wandering around.
Please let me know if any additional information is needed. I've included the crash log and server host information such as host OS, java version, forge version, installed mods, datapacks, and the startup script.
Arch Linux LTS (5.15.85-1-lts)
pixelmon_breeding_30min_v1.1.1 (Custom datapack to reduce breeding time for a small server)
pixelmon_custom_shopkeeper_v1.0.0 (Custom datapack to add a mending book vendor)
Crash Log: https://pastebin.com/cUaPSaUK
Startup Script: https://pastebin.com/r5s13VaR
If necessary I can upload the datapacks. The only files they touched are: