I found this issue happening over and over again, most frequently when I kill Plague Doctor, but it also happened now in just totally random moment, when I was building a wall.

Here is log file of the crash. I'm using Forge 1.20.1 (47.1.3) and latest Rats version as of writing:
latest.log

can you give me a crash report or 2 that this crash makes? I have a suspicion on whats causing it but the crashlog will provide me with some more important info I need

Oh yeah, I think all those three are related:
crash-2023-08-06_21.40.38-server.txt
2023-08-06-1.log.gz
crash-2023-09-14_11.04.01-server.txt
crash-2023-09-14_10.53.30-server.txt

I'm pretty sure this is an issue with additional entity attributes? They're assuming that the killer entity is present in one of their mixins, which they should be checking if it is present before performing any actions. I'd report it to them and see what they have to say. Feel free to link this issue in it!