You're right, changing the ".\GeneratedFiles\..." to an absolute path solved the above issue, but this cannot be an acceptable solution to the problem, right?
Furthermore, the build process seems to work now, but the program crashes when attempting to run it:
Qt Code: 
  1. Debug Error!
  2.  
  3. Program: C:\TestProject\Win32\Debug\TestProject.exe
  4. Module: 5.0.2
  5. File: kernel\qguiapplication.cpp
  6. Line: 782
  7.  
  8. Failed to load platform plugin "windows". Available platforms are:
  9.  
  10. (Press Retry to debug the application)
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Retrying gets me to "free.c" in mvscr100d.dll, where apparently some heap couldn't be freed.

I could not locate the file qguiapplication.cpp on the whole file system... Now I can either try to figure out why the relative path in MOC_DIR is not working and why changing it to an absolute path makes the remaining program unusable, or I can try to fix the heap exception thrown in a file which does not exist. But I have no clue about both. Seriously, what's going on here?