@kuzulis: Thanks for the hint to update to 5.6.0! Since it is some effort to update the embedded installation on the Qt target, I first tried the terminal example with 5.3.2, just adding /dev/ttyGS0 as a serial interface manually. Interestingly, transmission worked very well even after re-loading the g_serial driver.
So i had a look at my own qt program, again. First, very strangely, transmission didn't work at all, but after restarting the embedded system, doing a "clean all" on the project and built it again, everything now works perfectly also in my program... Very strange...
So I hope there has only happened something in the build directory and now everything is solved doing the clean and re-building...
Everybody, thanks a lot for your help!
Greetings,
Markus
PS: I also had to update my desktop qt installation for receiving the terminal example files (didn't find the bundled files on the qt site and my installation didn't provide them, also), but I guess this didn't have an effect on my cross-compiled application...