So I think I solved the problem.

After a update nothing worked. So I removed Qt and installed Qt library for Symbian. But this didn't work.
Then I installed Nokia Qt SDK and it worked. Don't know why.

Maybe the problem is not with Qt for Symbian?
Of course often I make some wierd stuff. But for me there is a lot of potential to improve Qt for Symbian (or mobile devices in general). I'm shure that Nokia is working on that and I'm looking forward to feature releases.

Thanks anyway
Luke