Outlook-Plugin session timeout

Hello,
we are using SugarCRM 6.5.16 and are sometimes experiencing problems while archiving emails from Outlook to Sugar: The email gets archived successfully but the team is not set to "global", instead it is set to the team-id of the user that is archiving the mail.
The problem occurs sporadically, but when it does, the logs are indicating a problem with the session-lifetime:

sugarcrm.log on the server:
Thu Feb 20 16:24:28 2014 [17256][-none-][FATAL] SECURITY: The session ID is invalid

Log of the client:
2014-02-20 16:24:27,074 [VSTA_Main] INFO    SugarOutlook.Soap.SoapAPI [(null)] - Sugar Web service - Session timeout   : Login attempt failed please check the username and password

The email was archived successfully at 16:24, but the team was not set correctly.
Our php-session timeout is set to 1 hour. Is there something else we could try except increasing the timeout?