Step 1: Make sure quark's guaranteed zombie villagers on normal difficulty is enabled with this mod installed
Step 2: Have zombies attack a villager
Step 3: Crash

Here is the report: https://gist.github.com/mxnmnm/e009b62ab98975e499fc4deb9b6cd7ae

Dupe of #50
Closing in favor of the other issue, which is already being resolved, and contains more logs.

We are experiencing this issue as well, and right now our server is unplayable due to this.

you can turn off that feature in quark and it works