Hi kuzulis,
you are right!
As mentioned above, I rebooted with kernel 3.13.0-62-generic and the serial-readAll works. Back to Kernel 3.13.0-65-generic the problem still exists (surprise, surprise,...).

So all this has nothing to do with qt. It's really a kernel bug.
(Never had such a nasty bug affecting me since the last 12 years...)

Once again - Thank you very much!