I am using 1.15.2. Here is the log: https://pastebin.com/KPR9YVu1
That also includes an error from earlier in the log. I thought it might be relevant.
I was testing a datapack at the time if that matters at all.

Do you have the offending datapack? If so, can you please upload it here?

datapacks are unlikely to have much to do with it, but link it anyways jic

Here is the link https://github.com/importnetminecraft/MissileWars-2. I doubt it will be much help because its big. I suspect the crash has to do with using commands to teleport fake players through dimensions or something like that.

yes - currently players can only be moved between dimensions once - that raises an exception in logs, but doesn't crash the game. Attempting to move them twice causes crash.

Also - dimension handling has changed in 21a again, so its an issue that needs solving twice.

very likely fixed in 1.17 by rewriting handling of entities.