I had originally set up openvpn without tls-auth and it worked fine with the N810, a Kubuntu client and an XP one.

With the tls-auth option, the OpenWRT server is complaining that HMAC authorization has failed. Double and triple checked both client (tls-auth ta.key 1) and server (...0) but only getting rid of the option results in a functioning vpn connection.

Either the openvpn server for OpenWRT or the openvpn client for Maemo have some trouble dealing with HMAC, I'd suppose.

HTH,
Andrea.