It is a real shame you upgraded from 1.21 - probably 99% of people on this board would pay a small fee to go back to that WORKING firmware...
As for the multicast, I have a couple of suggestions. First of all a reset to factory default then setup the router again (cofig restores after reboot have been buggy), also, try playing with SPI in firewall, try putting all your machines on fixed IPs handed out by the router (at least minimises other problems and probably adds stability), check out your QoS settings (especially after the latest firware) and see if adding a setting for your port will help - for mine I just have HTTP and WMC enabled, but for multicasting on a particular port it might be helpful to add more. Also, what is the port for multicasting? That really interests me. Is something else conflicting with it on either your internal network or on NAt translation? What's the port range for the multicasting?
Just a few ideas, though I know not much about it
