I found my mistake. During troubleshooting I must have turned off "AirGroup Location Discovery". After enabling it, everything worked fine:-) When roaming to the AP-Group that the AppleTV is located in, it became discoverable in a matter of seconds. However, after roaming back to the other AP-group, the AppleTV was still visible for a few hours due to mDNS caching.
The bluetooth tip was very helpful too, thanks a lot for that!
kind regards
Mikael, Denmark
C|EH, ACMX#340, ACDX #305, ACSP, CWSP, CWAP, CWDP, CCNA:Wireless