The Qt libraries are probably NOT the problem here. If you want Qt and do not want to compile it yourself, feel free to use his. They are LGPL. The problem had something to do with installing it. The launcher is different for fw 5.1.0, and perhaps a file got replaced. I will just reinstall the new launcher (no biggie).
Here is source code and build instruction for qt (and a sudoko demo program) on the DX (perhaps it can be adapted):
http://www.griffin.net/qt-and-sudoku...ndle-2-dx.html