Unable to access shares using VPN (L2TP over IPSec )

Hi Could someone tell me how to get a fix for the following issue

I am doing VPN into my network using L2TP over IPSec into the SBS server. I have a Cisco VPN server

It uses different account to connect to the cisco VPN does not use domain accounts
When i try open shares
\\<hostname> i get access denied , user does not have permissions
I researched and added Authenticated Users into the Builtin-->Windows Authenticated Access group (something like that ) ..then it gave me the user account was not found.

As per the netmon trace , its giving me KerberosV5 error and it is passing the VPN account credentials when it tries to access the shares. Thats why it is getting access denied. Is there a way to pass domain credentials instead of passing the VPN account credentials. It takes in this settings automatically. Please share some of your thoughts.

Thanks You.

PS: Able to access shares via FQDN and IP address also was able to map a drive using the net use command
net use z: \\<hostname> /user:domain\username password
Command executed successfully so definitely it is a problem with those vpn account credentials being passed on