QCoreApplication::instance() is NULL

[SGaist]

Since I don't know if you are using a shadow build, are you using the same names for your static debug release libraries ? Are you putting them in the same folder ?

If so you might be linking to a release version of your lib accidentally which in turn would pull the release Qt libraries.

[pullmoll]

After posting to Qt development ML and getting a reply, an even simpler solution is to just

CONFIG -= debug_and_release

in the *.pro files of the staticlib submodules. I don't know why, but debug_and_release seems to be a default. My project, developed with QtCreator since Qt4, doesn't define this flag.

[pullmoll]

@SGaist said:

If so you might be linking to a release version of your lib accidentally which in turn would pull the release Qt libraries.

I'm using QtCreator and have separate shadow build directories for debug and release. The culprit was the debug_and_release flag in CONFIG which I have to remove in the *.pro files...

[SGaist]

Still a bit strange but glad you found a solution :)