Outlook Calendar not showing appointments correctly

[F.A.J.D.]

I have Outlook for Office 365. on a Dell desktop.

After deleting a load of old contacts and then archiving my remaining contacts and emails, I now have the following odd behaviour in my calendar.
When I enter a new appointment into my calendar it shows correctly.


When I switch to the Outlook Today view the new appointment shows correctly.
When I switch back to the monthly calendar view the appointment is not shown, except for repeat appointments.
I switch back to the Outlook Today view and the appointments are correctly shown.
If I try and enter a new appointment in the calendar view that clashes with an existing entry even though it can not be seen I still get a pop up message warning about the clash.
The appointments synchronises correctly to my tablet and is shown correctly in all views.
I have tried a repair of Office, that didn't work, I have tried a uninstall and reinstall of Office that didn't work.
Has anybody any ideas.