Multicast (IPTV) and C470IP VoIP issue

[Builder71]

Recently I had to switch from bridged IPTV to routed IPTV. (My provider stops supporting bridged IPTV.)
Effectively this enables multicast traffic on my ASUS RT-N66U router ports.
I do have IGMP Snooping enabled, which should only send multicast streams to hosts that request it.

Funny thing is that if I plug the ethernet cable, of my Siemens Gigaset C470IP, into a random router port all IPTV boxes start stuttering like crazy.
If I plug the ethernet cable of the C470IP into a switch, directly connected behind the ASUS RT-N66U, all keeps working fine.
Previously, when I used bridged IPTV I could connect the C470IP directly to the ASUS RT-N66U router without problems.

Is it a known issue the Siemens Gigaset C470IP does not play nice when multicast traffic is possible on the ASUS RT-N66U router?
Or is IGMP Snooping not working properly while enabled on the ASUS RT-N66U router?

 

[Vexira]

Snooping should be fine is the gigaset up to date?

 

[Builder71]

Yes, snooping should fix it.
The gigaset is up to date, but the last update was ages ago.
(Firmware version: 022270000000 / 043.00 / 12-2010)

Would love to hear from anyone if they have seen the same issue.
It's a very old gigaset, but other than this still fully functional.