QtCreator - Compiling issue - External source files

I have a Qt project, accessing another cross-platform (boost) project on my disc. Adding the header includes does not seem to cause any problem.

#include "../../Visual Studio 2015/Projects/..." //Header file down the road

Adding existing source files to the sources folder in my Qt Project works also without a problem, the files are found and I can open them. I believe the files are not correctly compiled - if at all - as I get a linker error, telling me that %sourcefile%.obj could not be opened. (not created)

LNK1104: cannot open file 'debugError.obj'

I tried copying the content of Error.cpp into a new .cpp file created in the Qt project directory. After that the error message jumped to the next source file. I could now do this for all source files, but this seems to be quite... unhandy. Changes in the original project won't affect the Qt project then.

Does somebody know the problem / got a solution to it ?

I checked this question, answer and comments already, but that did not seem to fix the error or change anything.