How to fix 500 - Something Went Wrong, We couldn't find this mailbox error?

Question

Apologies in advance - I'm a new admin with little experience in licenses.

We have a shared mailbox between three people with full access. We haven't changed any settings, and yet one of us can no longer access the shared maibox via Outlook for desktop or switching between mailboxes in the web version. He can access the mailbox through "Open Another Mailbox" on the web app, but receives the error below when trying to switch accounts.

In his outlook the mailbox is visible, but when you attempt to expand the mailbox, there's nothing to expand.

No settings or licenses have been changed recently. Mine was added a few weeks ago in lieu of another member of the company who left.

The shared mailbox has a license for Power Automate and Exploratory, and the user attempting to access the mailbox has Business Basic, Business Voice, and Power Automate - the same exact settings as mine, which are working just fine.

Any help would be appreciated alongside any helpful tutorials to get my head around license distribution and permissions...