VPN Map network drive problem Windows XP

I have vpn connection to a univ.
I have 3 folders mapped on uni servers.
1 of the folders is only accessible if I bypass my router and connect PC directly to cable modem. Otherwise I get ‘unexpected network error occurred’.
Other 2 folders available regardless of whether connected via router or cable modem.
Router is belkin. Account details same for all folders. Troublesome folder could always be accessed via router connection before uni moved it to new server.