I have a similar problem with a RUTX08 set up as a UDP server on a TAPinterface with a simple static key.
This works perfectly with the same config on many RUT230, RUT240, RUT900, and an early RUTX08 on FW RUTX_R_00.02.00.2.
But on a new RUTX08 with the exact same config on the what i believe is the latest FW I can't get it to accept connections.
I have been monitoring the log with logread -f and noting happens. (The firewall is open for both TCP and UDP)
The thing is that it works flawlessly if I switch the config to TCP and in the client config on the PC to tcp-client. But I would much prefer to run the VPN on UDP for the reduced overhead.
Any idea what could cause this ?