Several users are reporting a calendar error: "Could not read the calendar" when they switch to the Calendar or to the Inbox. The appointments display properly and the usual fix for this error – reset the view – does not fix the problem.
Turning off the To-Do bar calendar reduces the number of times the error dialog comes up. Keeping the calendar open in a separate window so you don't need to keep switching between the Inbox and Calendar will also reduce the number of times you see the error message.
In the meantime, if you are affected by this bug and the error is too annoying, you can roll back to an older version of Office.
To rollback, type cmd on the start menu, right click on the Command Prompt, choose Run as Administrator then copy and paste the line in the command prompt. (This is a March 30 2021 build)
"C:\Program Files\Common Files\microsoft shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.13901.20312
For more information about rolling back office, see