One user can't connect to Exchange via Outlook.

One user can't connect to Exchange via Outlook.
Outlook 2016/2019 & Exchange 2016

Few days ago the client was unable to setup an iphone for the user/mailbox.
Exchange Event 1053 Exchange ActiveSync doesn't have sufficient permissions to create the ...
Followed:  KB https://support.microsoft.com/en-us/help/2579075/activesync-users-cannot-synchronize-an-eas-device-for-the-first-time-i
Resolved the issue with the iphone setup, but next day Outlook started to prompt for credentials.

Repeated efforts to setup the account/re setup have all failed.  Tried different machines and versions of Outlook - no luck
Always repeated password prompts & then fails.   Tried to manually configure.
OWA access works just fine
Same workstation & logged in as the user in questions - Can create an Outlook profile & connect for other users.  
Went into AD, user, advance security & reset to defaults.   Made no differences.
Exchange mailbox hasn't changed.  Active sync/OWA enabled.

Sort of stumped short of the nuclear option...  any ideas?
Thanks!