Any idea on why this only happens in servers and not Single Player?

Loading new Nether chunks seemed to have a high chance to crash the server.

https://gist.github.com/Aeron216/e3176114d057179cde30eb53e0302996

https://gist.github.com/Aeron216/7a6f59ab8da9352e372c63b6e0e33a33

https://gist.github.com/Aeron216/39da4ddd76789b6bd8df0addc3ab9b9c

One with no one online:

https://gist.github.com/Aeron216/248a0c01b3ab1c3c786325854a053ede

Seems to happen when loading and unloading certain chunks, because the moment everyone leaves the server instantly crashes the same way. Might even be a completely different mod at this point doing bad things so my apologies in advance.
I'll continue with my tests.

Might be Nether Portal Spread, had this issue back then in 2019 with it:

https://gist.github.com/Aeron216/1241c04796ac049d167bc5a10a3010a4

https://gist.github.com/Aeron216/4de2b8b409eef88aa7fe9104a9665cb5

This issues started soon after we opened our first Nether portal. I'll have to test if for some time without it. Hopefully this is the culprit.

I just assumed foamfix might have been it since it was the first thing I saw being mentioned.

I havent been able to reproduce the issue ever since I did something about Nether Portal Spread and its library Collective.

Closing this now.

I still get this even though its relatively uncommon:

https://gist.github.com/Aeron216/1052a1f9a292f03586acf92c36f9b30a

Same type of crash, seems to happen whenever the overworld unloads. As I said, it is relatively uncommon so I have no real way of replicating this.