Hi @B2theB ,
Great to know that it worked now and many thanks for sharing the solution with us!
By the way, due to a recent change in forum policies, now the question author cannot accept their own answers. So I've written a brief summary of this thread for quick reference, and if you don't mind, you can click the "Accept Answer" button below the post to make it easier for other community members to see the solution when reading this thread. Thanks a again for your valuable sharing!
Exchange 2016 - on-premise, Outlook 2016 fully patched and the Latest version of Teams.
One particular user's Teams Calendar remains blank and does not display any entries from his Default Outlook Calendar
"We created a test account and re-ran the test. It came back all green but what we did notice was that the mailbox had a false value for being an O365 mailbox. (which is correct as we are On Premise). So just to double check we ran it on the account with the issue which listed the mailbox as being in O365. (The value for Office365 mailbox being true).
So checked in the exchange admin portal online and could see a duplicate mailbox in 365. "
"We then followed the process in https://techcommunity.microsoft.com/t5/exchange-team-blog/permanently-clear-previous-mailbox-info/ba-p/607619 to clear up this mailbox.
Got the user to sign out and back into Teams and it now works, re-ran the test and it shows the mailbox as being on-prem."