What happened right before the crash (that was unfixable by restarting the server/relogging):

[In a multiplayer server world, only 1 player (me) present.]

Two villagers entered the nether, I entered the nether in an attempt to get them out of the nether. Upon loading the nether's terrain, server crashes. Villagers work fine in the overworld. The two villagers were born in 19w09a by breeding. Restarting the server and logging in again (as myself, as I was already in the nether) instantly crashes the server again. Further trials are unnecessary I think.

Jira is unable to attach my server crash report "due to missing token" at the time of creating the issue so I am uploading it when it is created.

Two crash reports are attached, with my limited knowledge and experience of looking at crash report: "Entity's Vehicle: ~_ERROR~ NullPointerException: null" may be the issue.