Funny enough everything works normal again.
The problem prevailed for 2 days. I restarted my computer several times. Restarted the Slingbox. Nothing changed the problem. The video was there for a short time and then went away but the audio stayed. The Sat receiver was working normal when I plugged it directly into the TV.
So I just gave up, posted my problem here and waited. I didn't restart the laptop nor the slingbox afterwards. Now everything works perfect again.
I guess the fix to this problem is waiting ;-)