Hello,
Did you restart your portal after modifying the ume property for logoff ? Following the documentation (http://help.sap.com/saphelp_nw70ehp1/helpdata/EN/9c/35004157acef6fe10000000a1550b0/frameset.htm), you have to provide the full url including host and port.
And, normaly, if you do not specify a logoff url, the users are redirected to the logon page.
Regards,
Nicolas