Display troubleshooting - how to test whether the service account can connect to the mailbox / meeting room

You can test whether the service account connected to Display can connect to a meeting room.

Go to microsofts test site here https://testconnectivity.microsoft.com/

Choose "Service Account Access (Developers)" and press "Next"

Fill out the form and press "Perform Test"

The test will tell you whether the service account can connect to the mailbox. If it can't, you need to update your exchange settings.