I digged a little around trying to find a solution.
In /etc/avahi/services I (only) found four defined services: afp.service, rsync.service, smb.service and website.service.
I deleted those files and ran omv-mkconf avahi-daemon. Those four files were created, my web gui still shows all services as unchecked / deactivated for discovery.
But now I least my afp service is found by my OS X system. (SMB not, despite the fact that the smb service is also running and configured.)
So, something is working. But I still can't deactivate/active service discovery via web gui.
I'm not really sure how to reset the weg gui, kill the gui's chache are whatever is responsible for that display error.
PS: Meanwhile I updated to 0.5.30.
PPS:
The rsync.service file looks a little bit odd. I won't file a bug report for that right now because I don't know whether this only applies to my already ... faulty system or is a general bug. Can someone chech his/her rsync.service file and tell me if it looks like this?
<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
<name replace-wildcards="yes">%h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync %h - Rsync</name>
<service>
<type>_rsync._tcp</type>
<port>873</port>
</service>
</service-group>