No multicast (IGMP reports) on VPN connection

Got an application that listens to (a number of) UDP multicast data streams.
Application works correct on XP and Vista.
On Windows 7 the application also works correct if the PC is directly connected to our (private multicast) network, only not if a VPN connection is used to the network. Th VPN connection itself works fine (i can ping the host machine) but no multicast is received.

Used Microsoft Network Monitor to trace the mulicast messages.
If the PC is connected directly to the network, IGMP Report messages are send if the appliaction is started.
If the VPN connection is used, no IGMP Reports are send. IGMP queries are received from the router but the PC doesn't response with IGMP reports.

Disabled the firewall but this didn't solve the problem.

Any suggestions?

PPTP VPN connection / UDP Multicast

I don's see why i should add a route
The route informations shows a route to the 224.0.0.0 range (the IGMP report should be send to address 224.0.0.2 for thes routers on the subnet).
Th route information is also the same as on a Vista PC that works correct.

A Cisco router is used as VPN server.
There is no difference between a Vista PC that connects to this server and the Windows 7 PC except that the Windows 7 PC doens't receice any of the multicast messages (apperently not because the Windows 7 PC does't send any IGMP reports)

A cisco 3845 is used as server and the Windows VPN client is used

This might help:

Enabling IP multicast: Networking and Communications

The following netsh commands display multicast group tables and troubleshooting information for interfaces.

• netsh routing ip show mfe displays the entries in the multicast forwarding table.
• netsh routing ip show mfestats displays packet statistics and input and output interface information for multicast forwarding entries in the multicast forwarding table.
• netsh interface ip show joins displays the multicast groups locally joined on each interface.