Did you run both of the following two commands as written below on the Exchange server and wait 24 hours to see if it resolved the issue? User mailbox data is collected only once a day, so it will not update immediately.
winrm set winrm/config/winrs @{MaxConcurrentUsers="30 of concurrent users"}
winrm set winrm/config/winrs @{MaxShellsPerUser="30 of shells per user"}