I cannot open any additional mailboxes - the button is greyed out - and one of the mailboxes for which I definitely have permission (I can open it under my account on Office 365) does not open when I use Outlook 2211

The other one I have permission for opens automatically.

I have tried removing and re-adding the permissions with a time delay between each change, but to no avail.

What is the matter with it and why is the option greyed out?