Sysadmin reported back with a possible solution, and it worked!
• Historically, there are two ways that calendar sharing has been implemented, which, for the purpose of differentiating them, are referred to as the “old” approach and “new” approach.
• The new approach is currently available for sharing calendars with view or edit permissions, but not with delegate permissions.
• You can use the calendar REST API to view or edit shared calendars only if the calendars were shared using the new approach.
• You cannot use the calendar REST API to view or edit such calendars (or their events) if the calendars were shared using the old approach.
If a calendar was shared with view or edit permissions but using the old approach, you can now work around the error and manually upgrade the calendar sharing to use the new approach. Over time, Outlook will automatically upgrade all shared calendars to use the new approach, including calendars shared with delegate permissions.
To manually upgrade a shared calendar to use the new approach, follow these steps:
1. The recipient removes the calendar that was previously shared to them.
2. The calendar owner re-shares the calendar in Outlook on the web, Outlook on iOS, or Outlook on Android.
3. The recipient re-accepts the shared calendar using Outlook on the web. (It will be possible to use other Outlook clients soon.)
4. The recipient verifies that the calendar has been re-shared successfully using the new approach by being able to view the shared calendar in Outlook on iOS or Outlook on Android.
So after i removed the shared calendar that was shared using the old method, then reshared and accepted it using web instead of the Outlook client it all worked fine.
So now it’s just a question of waiting on MS to fix this massive issue
More info on the known issue