Calendar sharing is no longer supported in Outlook.com. It started as an upgrade and temporally break, but half a year later, after contacting a Microsoft support, nothing has solved and the loss of calendar sharing seems to be permanent.
It started quietly without any warning from Microsoft side. All calendars in the Outlook Calendar app on Windows 10 Mobile stopped to synchronize with Outlook.com service. The reason was that broken Microsoft account. The operating system told that You need to fix your Microsoft Account, but after doing that nothing happed and everything was still desynchronized. Few days later the issue was solved, but all contacts lost profile pictures.
I have been searching what happened. The HoTMaiL, later knows as MSN Hotmail, then rebranded to Windows Live Hotmail, then to Windows Live Mail and finally to Outlook.com, have been using the largest SQL Server instance in the world. Relational databases aren’t designed to be scalable. In May 2015, Microsoft announced it would move the service to an Office 365-based infrastructure.
The problems with shared calendars syncing were expected, but no one bothered to inform end users. The interruption was expected to happen during upgrade process only. The sharing should be working again when all users will be migrated. But it did not happen.
The desktop Outlook can communicate with the server with at least 4 protocols. The first is POP/SMTP, the second IMAP/SMTP, the third is Exchange ActiveSync and the fourth is Microsoft Exchange. The first two options, POP, IMAP and SMTP, are very known and clear. I don’t know the technical difference between Exchange ActiveSync and Microsoft Exchange, but the Exchange ActiveSync was used by Outlook.com before the upgrade and the endpoint was eas.outlook.com. The Microsoft Exchange is used by Office 365 and so by Outlook.com after the upgrade. Microsoft was sending emails to users connected by the Exchange ActiveSync they need to reconnect to Microsoft Exchange. This is trivial task and I did it as soon as I noticed my account been upgraded.
After I reconnected to the new protocol I wasn’t able to edit shared calendars. Even when I created a new calendar and shared it to other already upgraded users, it did not solve anything. The shared calendar was read only for anyone but the owner in Outlook 2016 version 1701 build 7766.2060 for desktop PC. Editing the calendar was possible only via web browser. The Outlook Calendar app for Windows 10 Mobile version 1607 build 10.0.14393.693 is unable to save events to the shared calendar. It pretends to work, but the event is never shown in the Outlook.com calendar.
I have contacted the Microsoft support to solve the problem or warn about an issue (case number 1367529784). At the start form, there is a statement: The more you write, the more we'll be able to help. That’s good, but the text area has a very low limit of the text length so I wasn’t able to precisely describe my problem. When I reached a very kind person from the support, I had to explain everything again, because he didn’t receive the description of the problem I wrote earlier. I spent one hour by describing a demonstrating the dysfunctionality and the result was simple. The official Microsoft solution is to downgrade back to the eas.outlook.com and use Exchange ActiveSync again. When I explained that this protocol is going to be discontinued soon I was reassured that this issue will be reported internally to the product group. Few months later the Exchange ActiveSync was really discontinued, but the shared calendar is still in read only mode regardless my effort make it work again.
I would expect that Microsoft will be clear about this issue and tell when the migration will be finally done and what should be still working and what is not expected to work anymore. Instead, the Outlook team is focusing more on Mac and Google users.