That's an App Server Config error. It's trying to display the Sessions tab.
1. Stop the Application Server and also close out of App Server Config if you're in it.
2. Rename the Sessions.pvk data file in ..\mas90\Home\Lib\_Appserv folder
3. Start App Server Config again. You should see empty Sessions tab since the data file is brand new and empty now.
4. Start App Server again.
Hope that helps.