Situation:

Display port is connects my display and my AMD card
HDMI port connects my sound system (deck) and my AMD card
After loging in, Plasma switches to the HDMI "display", making my monitor dark. Any tips how to avoid that?

Resolved the issue. It seems that "splitting" sound and video this way causes several problems. The drivers (?) do not know what is what, so it is not just the wrong signal at the wrong port, but generally a very unreliable solution.

Do not try it at home kids!