Thx for the quick response!
We've upgraded to 1.16.3 now and the problem vanished!
Appearently the mods we added didn't work with the rest of the pack anyways. But we managed to get some of them running.
The large file came from the server host Nitrado.net. Maybe they incorporate stuff to make it work with their systems?
We were on 1.16.1 because the modpack was still on that. But now it's updated.
What added to this was the launcher we used. At first we used the Twitch Launcher which worked only half of the time, then MultiMC which didn't work in MP at all and then we switched to the standard MC Launcher and with the upgrade everything worked!
So after all it was an error on our side, thanks anyway, sorry for thinking your files were faulty
Best wishes
TheDoooden