since few days, I have an error for a single user in VBO :
Error: You have exceeded the available concurrent connections for your account. Try again once your other requests have completed.
There are only 5 connections allowed to a specific mailbox. So at that moment multiple connections are happening at the same time. I am not sure how to check this but for that specific mailbox maybe multiple (or lots) of devices are connecting? I think the ConnectionByClientTypeReport cmdlet might help you