Client sees video files but none is playable

Hi, I didn't find any posting regarding a similar effect:
I've got a windows home server 2003 with the latest update and choose to install serviio instead of using the "on-board" Media Connect (MC)- because MC doesn't serve .ts-Files.
My client is a Kathrein UFS 912 HD Sat Receiver, which is capable of talking UPNP.
So, everything seems to work at first view, all video files are shown by UFS912 ... but - they are not recognized as playable videos.
Counter-Check: If I switch back to media connect as UPNP-Server, my videos are only shown, if I rename them to e.g. .avi - but then, they are always recognized as videos (which can always be seen by an icon beside the video title) and they are playable without any problem.

With serviio, it's no matter if I rename a file to some other extension, and it's no matter which profile I choose, none will make my client recognize the videos as being something playable.

I suppose there must be a quite dumb mistake I made, but I have no clue ...
Sorry I can't provide a log file or more detail on my serviio version at the moment, but I will add it later ... if my mistake isn't so obvious that some serviio expert sees it immediately.
Thanks in advance!
IngO

Update: now I tried to gather some information by means of intel upnp tools, but:
- device scan does not show my UFS912, thus I cannot add anything of value.

The "device sniffer" shows two packets coming from UFS912 about once in a minute, announcing it as a "upnp root device"; device scan seems not to recognize that.

More information a little later; I'll try to add a detailed serviio log.
Regards, ingO