Shared mailbox cannot be pinned to Favorite

When I want to rightclick on the inbox under Shared with me in New Outlook the pane doesn't show up - so I cannot pin it to Favorites. There is simply nothing, just a very short shadow of the pane.
I tried to delete and reinstall the new Outlook app but to no avail. Does anyone have the solution her, I would highly appreciate your help.

Kind regards, Astrid