I've been programming with Ot for a week. Everything was fine, but since yesterday I can't set up a new Qt widgets application or include a file. Every time I set up a new project it looks like this: Screenshot Editor
I get the error message that eg QMainWindow cannot be found. I also can't include other files like string, iostream, ... . I haven't changed anything and with the same configuration I can open & build old projects or a C++ application without qt. Does anyone have a solution to fix this problem?