Crash Log (REQUIRED)
Useless crash report-
https://paste.fedoraproject.org/paste/TQHGYXFFDDWxfOXcJ5nDHQ
The actual stacktraces are in the logs.

Full log-
debug.log
latest.log

The happened before but dismissed it as a fluke, happned again so reporting.

Versions:

• Forge Version: forge-1.14.4-28.0.45
• MC Version: 1.14.4(this question is redundant ;)
• Mod Version: betteranimalsplus-1.14.4-7.0.2.1

Caused here https://github.com/itsmeow/betteranimalsplus/blob/1.14/src/main/java/its_meow/betteranimalsplus/BetterAnimalsPlusMod.java#L80
We may need a non-parallel forge event for collecting biome features.

I don't understand how I'm concurrently modifying anything. Wat.

Because that event is run concurrently with other mods which can also do the same thing, resulting in such an error. You should schedule feature adds to the main thread using DeferredWorkQueue

Thanks for the help @Shadows-of-Fire :)

Fixed in 7.1.0, out now