Here is another issue I encountered during my Exchange 2016 migration. A hand full of users have been getting knocked out of public folders with an Event ID 9646 and they have exceed the max of 32 session objects. What the hell does that mean? Why only a few users?
Investigating Event 9646
The Monday after I migrated public folders to Microsoft’s new “Modern Public Folders” a hand full of users began calling me they were unable to access them. Their specific error was:
“Cannot expand the folder. Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.”