While adventuring in a nether fortress, there are mysterious crashes. The description is "Ticking entity" and the stacktrace length varies.
Whenever I log back on, I create another server crash with the same error.
(This bug, or a related one, also happened in the overworld but did not repeat itself when I logged back on)

However, loading the chunks from a second userdata file or with a friend will fix this (until it happens again)
This seems to be caused by mobs attacking or killing one another, and related to MC-91764 (which was marked as a duplicate of MC-91786 and "resolved")