Using New Outlook on Windows for personal email, calendar, and contact management
As a last resort and to my surprise I found a solution!
I have 4 email accounts 2 x 365 and 2 x IMAP and I was struggling like everyone else to get an overview of all 4 in the Favourites box top left. I deleted the defaults (I'm sure you know how to do this) leaving me with one 365 account [Inbox] and one IMAP account [Inbox] and then I closed down Outlook. Started Outlook again and added a second 365 [inbox]. Shut down Outlook again and after restart I added the second IMAP [inbox]. I now have all four accounts' [inbox] in the Favourites box.
No skill, just persistence and good luck.
Even more frustrating was any other folder than an [Inbox] I could add without a problem.......