Arrak

We noticed that some On-Premises users can open calendars from Exchange Online but this doesn't seem to correlate to the Exchange Online users who can open from calendars from On-Premises.

Looks like it. Problem is resolved for me and several other accounts but majority of our organization still seem to be suffering from the problem. I also noticed we still can't open any Exchange Online calendars from On-Premises OWA.

Our support is saying that it is "by design" and they are on the verge of closing the ticket. We just had it "forcefully" escalated by our Premier-contact.

Good to hear at least something is happening on your end.

In OrganizationRelationship you can set the default level from Free/Busy to LimitedDetails. ( for example we have FreeBusyAccessLevel : LimitedDetails). Also in ECP, you can configure several different levels of access to calendar for cross-forest (or external) calendar access). But if we get same answer you got, it is kind of a deal-breaker for us because we were told by MS consultant it is supposed to be possible to open cross-forest calendars with Free/busy or with LimitedDetails. I would understand if the calendar would open with Free/Busy status shown only, but currently you can't open the calendar at all anymore. Most of our users are Outlook users so the situation is not worst possible. I fear the OWA change is just the beginning and Outlook stops working too. We still haven't heard anything from the support.

On a interesting side note, we did network tracing with browsers (IE/FF) and noticed that https://outlook.office365.com/owa/auth/errorfe.aspx is returning httpCode=500 (which is InternalServerError) when you try to open crosspremises calendars. We haven't heard from MS anything yet...

Yes it has. Users could open each other calendar (crosspremises) without any problems since we had hybrid set up. Until last week friday everything worked fine and then suddenly we started get reports from users that they can't access calendars if they are not specifically shared for them.

We have similar problem: O365 owa can't open any OnPrem calendars but scheduling assistant shows free/busy and OnPrem OWA can't open any O365 calendars. Everything works just fine when tried with Outlook. We have opened a premium support ticket but haven't got any answer yet considering this matter.