Are you sure those room objects are actually synced to 365?
get-recipient <room>
Do you see it via Exchange Online Powershell?
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Last week I completed a Hybrid Configuration setup between our on-prem Exchange 2016 servers and 365. I've migrated myself and a couple of other IT staff.
I did a Full Hybrid Configuration using the Classic Hybrid Topology.
So far everything appears to be working normally, however I no longer have access to any of the conference room calendars or mailboxes I had full access to pre-migration (all of which are still on-premises). Clicking on a conference room calendar in my Shared Calendars doesn't bring up anything at all (not even a blank calendar, it's like it doesn't even register that I clicked on it). I can click on on-prem calendars from user mailboxes and see their free/busy data, but not the room mailboxes.
I found an article indicating that the default permission on these calendars is None and that I should change that to Free/Busy or one of its variants, but looking at the affected calendars they're already set to Reviewer (Full Details) so that isn't the issue.
I looked at Organization - Sharing in the 365 EAC and it shows that sharing is enabled on the migration endpoint:
Please advise.
Are you sure those room objects are actually synced to 365?
get-recipient <room>
Do you see it via Exchange Online Powershell?