Forum Discussion
Odd Azure AD Connect Sync Issue
That screenshot isn't overly useful.
It might pay to check AAD Connect, specifically within the "Synchronization Service" Manager. I'd anticipate you'll find an error pertaining to this user's account in one of the synchronisation cycles (likely a DS, or Delta Sync, cycle).
My assumption from what you've written is that "mail" was nominated within the AAD Connect configuration manager as the joining attribute, which would explain why changing its value allowed you to progress - albeit in a not-so-useful manner.
If that assumption is accurate, then you would want to check for any on-premise duplications of that mail address through something like the following in PowerShell:
Get-ADObject -Filter { (mail -eq "problematicAddressHere@yourdomain.com") }
You can run into derived issues like conflicting proxyAddresses produced from the MOERA process but I'm just speculating again where it's probably prudent to start basic.
