Description of the issue:

Crashed. Not sure what exactly. Will try to ask the player that was online. But considering the crash I don't think its going to help.

Crashlog:

https://gist.github.com/Darkere/61dbbd75dd033a0852060b67621d9c1b

Versions & Modlist

forge-1.16.1-32.0.108
ImmersiveEngineering-1.16.1-3.2.0-115

Pack: Enigmatica 6 0.2.14

But wouldn't it crash on the other accesses then?

what the actual...
That call is in a synchronized bracket. That shouldn't be able to hit a CME.

@malte0811 do you have an idea what's happening here?

Blu, that access is synchronized. All other accesses (e.g. here, that one is probably causing the issue) are non-synchronized. So the synchronization only means that we can't ever have two threads simultaneously writing world save data.

No, why would it? All synchronized does is ensure that no two threads are in a synchronized section for the same object at the same time (the "later" thread has to wait until the"earlier" thread is out of that section). If you have one thread doing world gen and one saving data, the synchronized block in IESaveData might just as well not be there.