Hello,
after an extensive try & error session I found out that it depends on the resolution of the monitor connected to the display port. With a resolution of 1280x1024 I got an output but that looks terrible on a 24" monitor.
After playing around with the options of the radeon module I had success with
options radeon audio=0
I don't understand why because this option should disable hdmi-audio. Strange but it helped.
But I had to pay a price. I lost my dual-screen setup. KDE system configuration won't save my 1920x1200 (24" on display port) and right to it 1280X1024 (19" on dvi port) anymore. Now both displays have 1280x1024 resolution sitting on each other. I have to rearrange both displays and restore optimal resolution manually after each KDE start. Rule 1: do try & error step by step
Rule 2: remember your steps
I forgot to reenable kscreen2 service. Now eveything is fine again.