Yesterday one of my colleagues reported that she couldn't see free/busy info anymore in her Outlook client calendar. But she also reported that in Teams the calendar was unable to load.
I set up a brand new Windows 10 Enterprise Virtual machine, logged in to office.com and started the Teams web version with the same problem, no calendar in Teams. It seems to try a bit, you see an empty calendar grid form but then the lady appears telling that the calendar can't be loaded.
We run Office 365 A3 accounts so we don't use the free Teams where calendar is not implemented.