We are having this quite a bit now that we have added templates which are found in my The Adventures of halk pack -
https://www.curseforge.com/minecraft/modpacks/the-adventures-of-halk

Reproduced with only ruins and forge-
https://gist.github.com/ProsperCraft/6ade5a1c2b97533ef837117a3e2463bb

Log -
https://gist.github.com/ProsperCraft/2799326e6256620509a2d28b650c5a3b

server crash with full pack -
https://paste.fedoraproject.org/paste/ayx9xiz3HDsTE2TYj8kC~w

OK. It's probably because chunk generation is async now, and Ruins altering the world at an unexpected point in time ... i do have something i could try to fix it.

bb1afbf in a short test on a dedicated server with the templates you linked, it spawned a couple of them and didn't crash.

Seems to have cleared it up :)