IMAP Gmail popup in Outlook 365

[twinbytes]

I have an unusual problem I'm stuck on.  I'll tell you my setup, exactly what the problem is, and what I've tried so far.

THE SETUP
We are using Microsoft Outlook 365 and it has two email accounts.  One is using Google GSuite while the other is using IMAP, also via Gsuite, but since we can't have two go through the Google MAPI program, the second one is setup as an IMAP. 

THE PROBLEM
The problem is the second email via IMAP is prompting up constantly to login, as per screen shot attached, every time it tries to send/receive.  The main Google account works fine.

WHAT WE'VE TRIED SO FAR

• Setup email on another computer the account works perfectly fine.
• Deleted email address in account settings and recreate it
• Created a new Profile
• Deleted account and recreated on Gsuite admin console
• Tried server name mail.gmail.com as well as mail.googlemail.com which is apparently suggested as the new server names.

URGENCY LEVEL
High, it's so annoying the client has referred to using Webmail until resolved but absolutely hates Google webmail.  All suggestions are welcome.