Outlook Group Calendars - The operation failed. An object could not be found

Hello everyone,

We have an issue where a user can't use any of it's M365 group calendars in the Outlook client (for Windows) anymore. Once a new item should be created, we receive the error message from the screenshot below. The permissions are all fine, since on OWA the calendars are fully functional.

What we did so far:

- Recreate the mail profile in the control center

- Reinstall whole Office suite (includes Outlook)

- Upgrade Office suite from 32 to 64 bit

Finally the mentioned error message (unfortunately in german), but I think the english translation would be "The operation failed. An object could not be found.":

Maybe someone has an idea how to solve this issue?

Thank you!