Exchange Hybrid: Shared Calendar issues with Outlook

[Luke_IT]

Since approx 48hrs ago we noticed Outlook not receiving new calendar updates / meetings from on-prem Teams rooms calendar mbox in a hybrid scenario. The Teams room calendar itself receives the invitations and meeting schedules as normal but the people who created the event and invited the room do not see it in the Teams Room calendar they have added in their Outlook (though they receive the meeting invite on their email and personal calendar) whether they have ownership permissions or not. This has always worked fine until now. The only way I found to workaround this issue is to *disable cache mode* in Outlook. I tried removing the calendar and re-adding it to no avail. Removing the cached mode, deleting the cache files and re-enabling the cached mode will not resolve ths problem and the Teams Room calendar will remain all BLANK and not update: I verified the correct connection to the on prem endpoint and it is correct and no errors are reported (same connection with cache disabled of course). This points to an Outlook 365 operational issue or bug and not Exchange itself in general so I don't know if this is the correct group to point this out but it does involve Exchange Hybrid mode.

Steps to reproduce the issue:

Enable cached mode in Outlook 365 in a Classic Hybrid mode scenario. Add a shared (Teams Room in my case) ON PREM calendar. to your mbox in Outlook (you can additionally set ownership permission on the shared calendar but it doesn't change the result). Verify the absence of displayed events. Disable cache mode and restart Outlook, the events will appear.

[#2]

Teams rooms in onpremise exchange is not really supported as far as i am aware, to support full functionality you should use exchange online mailboxes.

[#3]

That is actually incorrect: I have been using Teams with on-prem Exchange mboxes for years and it is a supported scenario since day 1 as the only way to use Teams with on prem Exchange is de facto a *hybrid* configuration whether basic (just oAuth) or full-blown and once this is setup it makes really no difference whether the mbox calendar is on-prem or in EXO (if it were, no on-prem individual nor shared mboxes' calendars would work).
Please refer to the following link for more information:
https://techcommunity.microsoft.com/t5/microsoft-teams-community-blog/microsoft-teams-and-on-premise...

The problem here is not really related to Exchange per se but rather the client Outlook 365 in cached mode after a recent update. Non 365 versions of Outlook continue to work correctly in cached mode.

[#4]

oh you are not talking about Teams Rooms specifically, you are talking about a resource mailbox which u use as a teams calendar. I would suggest creating a ticket in your tenant, also if ur already in a hybrid mode why not just create a ad object on premise and the create the remote mailbox via powershell that way you won't have calendar issues.
this probably has to do with your inta organizational connector:

https://learn.microsoft.com/en-us/exchange/configure-oauth-authentication-between-exchange-and-excha...

[#5]

Actually I was referring to a Teams Rooms - in any case I have resolved with the above workaround. It's highly probable that the issue will be fixed in Outlook 365 in future updates as I'm positive I'm not the only one with clients in this scenario.

[#6]

Does it work now?

[#7]

I have not tried to revert back to cached mode on those machines with O365 (the ones with 2016 and 2019 work fine). At the moment they are working fine with online mode and I will leave them like that for the time being until I manage to have a maintenance window and try cached mode again if I see the Outlook 365 version has been updated.