An Office 365 Exchange Online user had a shared calendar he couldn’t delete from Outlook or Outlook on the web.
He received a sharing invitation from another user and the shared calendar was added as a subfolder under his calendar when he accepted the sharing invitation.
The user later shared his entire mailbox with my client but they couldn’t delete the previously shared calendar.
When he tried deleting it in Outlook on the web, the error message simply said "calendar can’t be deleted”. Trying to delete it in Outlook did nothing; no error message.
After some investigation, I discovered the problem: he had a calendar folder by the same name in the Deleted Items folder. I permanently deleted that calendar and was able to delete the shared calendar.