At times TeamSessions server application is unable to open notes database which results in this issue.
Steps to address this issue:
1. Open the itsessions.nsf database using Notes Client
2. Update ACL. Server should access the database as person having Manager access level (refer screenshot)
3. Next restart the “Sametime Persistent Chat Application” service from Services applet located under Administrative Tools folder in Control Panel. (refer screenshot)
4. Wait for 5 to 10 mins and test archiving using the TeamSessions plug-in client