I am not sure if Better Portals is the root of the issue or just the catalyst, but if you could find anything, that would always be an improvement on it's own.
crash-2019-09-22_17.55.50-server.txt
I'm not sure on how to reproduce this issue, it doesn't always seem to occur.

Looks like an issue with Cubic Chunks.
However, given BP severely messes with CC's chunk/cube tracker (1, 2, 3), it may very well be my fault.

I'd suggest bringing this up to the CC people in case they recognize it or can immediately tell what went wrong (the code indicated in the crash report isn't directly modified by BP, so I'm not yet familiar with it).
If they can't, then I'll have a look into it (assuming I can reproduce it somehow).

This is actually caused by CWG far plane view mod, go report it there. It's invoking vanilla generator on it's own threads which simply isn't going to work.