SSL VPN: no matching policy

Hello, I have an issue affecting randomly our SSL VPN users. The users are LDAP users. Sometimes they can login, sometimes not and sometimes after several attempts. Looking at the event log, I did notice that the reason was " no matching policy" . I thought it could be a bad password, so I went to my domain controller and tried to find a similar event for one of the users. But did not find anything. This means that the Fortigate did not even connect to the DC to ask authorization while when the user can connect successfully, I do see an event on the DC coming from the Fortigate. Fotigate version is v4.0,build0272,100331 (MR2). We also require Client certificate on top of LDAP username and password. Error: Log Number 3 Last Activity 2010-10-26 08:44:44 Level alert Subtype sslvpn-user Action ssl-login-fail Message SSL user failed to logged in User david.portal Cluster ID FG600B3909600928_CID Log ID 39426 Timestamp 2010-10-26 08:44:44 VDom root Device ID FG600B3909600928 Reason no_matching_policy Tunnel ID 0 Tunnel Type ssl-web Remote IP 81.43.116.213 Tunnel IP 0.0.0.0 Sent 0 B Received 0 B Thanks for your help