PPTP VPN and *SESSION credentials causing access to file share to not work

Hi all,

I have a problem with a client site.

They have a PPTP VPN configured on a Draytek 3900 series router, however the users are local to the Draytek and not linked via LDAP.

There is about 40 users, mostly running Windows 7 and they're all fine, however there is two users who are experiencing issues accessing the file share.
When they try and access the file share, they get an error saying Invalid username or password, and the only way around this is to open Credential Manager and delete a *SESSION credential which is created when the VPN connects.

I've taught the user to delete the *SESSION user on logon, however this isn't practical as a long term solution.

I have tried removing the PC from the domain and rejoining, I've tried the same credentials from another environment (eg from my work PC - and this works)

Any ideas on how to get the laptop to not save a *SESSION credential?

Thanks