Errors after sharing SYSVOL - 1 User can't access via WAN

Preamble - Since I upgraded to Symantec Mail Security (SMSME v5) at Xmas, one user has not been able to access our domain (for Outlook, etc) when coming in over the WAN, however, all works fine when he connects locally via LAN.

Exchange Server = Windows 2000SBS +SP4 & is a DC
Data Server = Windows 2000 Server + SP4 & is a DC

I have only been able to find one error and that was using DCDIAG...

"There are errors after the SYSVOL has been shared" - However all tests pass OK, FRS is enabled (all be it after repeated tries).

I tried using the other copy of SYSVOL, but no joy.

I have also uninstalled SMSME, but that has not fixed it either.

One or two internal (LAN) users have reported that, again only since Xmas, that they have been asked to log back into the domain randomly during the day, not all users and not every day, but that's it. Everything else appears to be fine, no unexpected errors in any event logs.

If the one remote user logs in via MoD secure Dial-up, he can gain full access to our domain, but if he comes in via MoD secure Broadband, he has no access to our domain, although he can ping the servers OK and access the WAN.

I have tried, on another laptop, connecting via dial-up and I can not get in at all, if I try from IE7 typing "(// domain.com / sysvol / domain.com)" (where domain.com=our domain name and / = backslash) I get a login box, but it won't accepts any user name, even Administrator. If I try the same from a command prompt I just get "Access Denied" I can however ping all the servers and connect to the rest of the WAN.

I don't want to do anything that might upset everything for the other 20 users, but I would like to fix this problem.

I'm on a steep leaning curve here, so please be gentle.

Thanks in advance.

Dave.