I was using it with the Terra mod which is still in beta so that's probably why.
crash-2021-05-07_19.18.52-server.txt

If you want to use terra with cave biomes you have to disable their cave carvers, because terra adds it's own. This is a terra issue, not a cave biomes issue, so this should be closed.

Yeah, this seems related to terra. I'll pass the issue along.

Can confirm the same issue.