Hi DINDOGERON,
This error message seems caused by the lack of the permission.
I’d like to check the permission on this shared calendar.
1. Connect to Exchange online remote PowerShell.
2. Run the following command to check whether other users have permission on this shared calendar.
Get-MailboxFolderPermission -Identity user@domain.com:\calendar
Please let me know the results after running the commands above.
Thanks,
Frank