User identification timeout in RDP session

Hi,

We've just started using a FortiGate firewall and have configured the FSSO agent on a DC.

We have come across an issue when RDPing to a machine (over VPN) that the user identification fails (i.e. no username/group name are displayed on the block page) and the entry is lost from the FSSO agent list of logged on users.

Disconnecting from the RDP session and reconnecting is enough to kick it back in, however, it disappears somewhere between 5-10 minutes later.  This behaviour can continue for a number of hours and is affecting more than one user on more than one PC.

Has anyone come across this before?

Thanks in advance,

Giles