Did you make a new config from scratch?
I flashed the router with the new firmware, reset to factory defaults, and then went in and set up all my configurations again like it was a new router.
Additional, after the new firmware version (1.20) I'm seeing my router not only not properly handle UPnP, but overall unstability. The router is freezing up daily now and becomes unresponsive. I can't even access the web utility. The only fix is to power cycle the unit. I have even flashed the router again with the firmware and done the above to verify that it wasn't some fluke.
From my experience, 1.20 has turned the DGL-4500 into a complete and utter disaster that needs to be fixed ASAP.