crash-2021-08-30_19.46.20-server.txt

It seems like your forge versions are all pretty old. Can you please try updating the version to at least 36.2.2?

I've gotten it to occur twice by going near the ocean and waiting so I believe the game's trying to spawn something but it can't and that causes it to crash.

I believe I'm seeing the same issue.

dungeons_mobs-1.16.5-1.0.10.jar

crash-2021-09-01_18.24.47-server.txt

Same here, I even tried to disable spawning using the configured mod, but it kept randomly crashing near water.

crash-2021-09-13_14.32.28-server.txt

Yes, if you are getting a crash in the log that says "NoSuchMethod" it is because of you not using the at least Forge 36.2.2 which has necessary features for newer versions of the mod