Logon unsuccessful

Dears,

some time when I connect to a corporate network via VPN and need access to a

shares I'm asked for an authorisation.

After the authorisation recieve this message

"Logon unsuccessful: The user name you typed is the same as the user name

you logged in with. That user name has already bee tried. A domain controller

cannot be found to verify that user name."

Some time I could access without any problem. Could someone advice what

could make the problem. It concerns only file shares. Other communication

like mail is working withou any problem.

Thanks

Jiri