qmake.exe not found or executable

I built my own 64-bit Qt 4.8.4 using MinGW-builds. Apart from additional problems with the compiler (https://bugreports.qt-project.org/browse/QTCREATORBUG-10584 , https://bugreports.qt-project.org/browse/QTCREATORBUG-10582), I encountered a problem with QtCreator, I couldn't track down properly:

The qmake.exe I built I can select in QtCreator and everything works fine. When I close QtCreator and reload it, It says the qmake.exe cannot be found or called, although the file path is correct. If I click again on "choose file" and select THE VERY SAME qmake.exe. Everything works fine again, until the next restart of QtCreator.

Is there any way, I can get more debug information on this issue out of QtCreator (e.g. what EXACTLY goes wrong, when calling qmake.exe), that can help to get more info on this problem?