Demystifying Hybrid Free/Busy: Finding errors and troubleshooting

"@C-Money the thest OAUth Connectivity  if you are using the Intra Org Connector may show aditional error detail."

OAuth Connectivity completes successfully for the user that is not working and the user that is working. So apparently OAuth is not the issue here.

"From my experience, socket error generally is due to Nertwork issues. Remote Connectivity Analizer, Checking from OWA if behaviour is the same as in Outlook for the affcted user and check from a computer that is know to work for Free Busy calls from other cloud mailboxes may be good tests."

I have tried this from Outlook 365 OWA and other PCs that I know free/busy works from and when I am logged on as the user with the problem the free/busy does not work under any circumstances. The message I posted before was results from the Remote Connectivity Analyzer. When I am authenticated on the Connectivity Analyzer with user A, it fails. When I authenticate with any other user with a 365 mailbox it passes. It only fails for this one user. I have compared all the on premise AD attributes and there are no differences that would cause this. I truly am at a loss.

Something must have changed either in 365 or on my on-premise environment. I ran the August Exchange security updates, I wonder if that broke something perhaps. But if that is the case I don't understand why that would effect this one user. Nothing makes sense.