I disabled the steam cloud (and logged at least out of my xbox live account), but didn’t turn off the steam overlay. I may try this out aswell next time. In the worst case, it doesn’t help either But I’m really curious, what’s the reason. I already wrote a support ticket. They recommended to install the game on another drive, but the issue still appears. Replied them again and now I’m waiting for their answer.
I think they need to provide a hotfix or a fix in the next major update. Because more people have this or a very similar issue. You are also forced to delete the scenarios ingame, which is weird.