S/PDIF Output not working after playing movie with Dolby Digital

Hi community!

Recently I had to reinstall my Mac mini (The white Early 2010 model). Since then if I play a DVD or any other video with Dolby Digital audio via the S/PDIF output the digital audio output does not work untill I restart my Mac.

While I'm playing the DVD or video the sound works fine. But if I stop the video and try to play another video or listen to music: I can't hear anything.

My A/V-Receiver also does not show a signal from the optical input. If I plug out the optical cable from the Mac so I hear the sound via the internal speaker, the audio is playing normally.

What is wrong with my Mac?