Thanks a lot, I would definately not find it out myself.
It seems that removing "\0"s in sprintf functions solved the problem 
Edit: I think I was a bit too excited that it worked... The issue still occurs if a print statement is added. I'll try to take a closer look at the compiler warnings, thanks anyway.
Edit_2: I tried to remove every occurence of F(), it seems to fix the issue but it's difficult to say whether it's "real" solution unlike adding_some_random_Serial_lines_fix...