Thanks. Unfortunately this is not what I have. My problems-window is empty and the text file encoding is already set to UTF-8. Must be something different. I definitely had a reason to be flagged as warning at some point in time but I fixed this and this /!\-Icon sticks there.
Any other idea where I could look?
EDIT: Sorry, I overlooked that the description column of the problems window was occluded. Now I see there are warnings that some include PATHs are no longer existing. I added some include paths (USB folder) and later I removed that again forgetting to also remove the references to the include paths. Will do that now and report back.
EDIT: Yes, it was that. The dangling include (USB) paths. Removing them also was a bit unintuitive.
Had to go into properties->C/C++ General->Paths and Symbols and delete the Include directories no longer valid. Anyway, problem solved. So the solution is to look into the "problems" view.