Hi,
short follow up to my problem.
I did a vanilla install of the stable version (with passthrough working) and updating to 7.90.010 (i originally had 7.90.009) with passthrough working as well.
I then did a update to 7.90.0.10 of my main installation and passthrough is working again....maybe some bug with 7.90.009?!
Long story, short.....update to 7.9.0.10 if you have troubles with passthrough.
Thanks for your replies, and thanks to chewitt.....I think you were the one who made big contributions to the ATV part of openelec.
I used it until HD sound became important to me.
Cheers guys!
KR,
Matthias