You could try soft-resetting the streaming network interface.
That said, in over a year of owning the Bartok, I’ve never once experienced any UPnP bugs where it stops responding. I’m guessing more likely something else is going wrong on your home network that gets “reset” when you reboot your Bartok (like a fresh IGMP Join request for example).
Did the problem only start happening lately? Is your Bartok connected to the same Ethernet switch as the UPnP Server? If you’re using a managed Ethernet switch, check if IGMP Snooping is turned-on, just disable that and see if the situation rights itself. 