Outlook constantly requesting user credentials off on-premises Exchange Server 2013

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For the umpteenth time, I have the problem of Outlook clients constantly prompting for credentials on opening and repeating on correct entry, though unlike previously where a deletion of the Outlook profile would remedy it, this is no longer the fix.

 

Background info:
* Fully On-premises Exchange 2013 CU22 (CU23 won't install due to failed Exchange 2019 installation last December) on Server 2012 R2. ** NO ONLINE OFFICE 365 INTEGRATION **
* Usernames are NOT the email address, the Active Directory originates from Server 2000 when username@domain.local was the norm...
* Affecting all Outlook clients from 2013 to latest Microsoft 365 builds 2009, both on-premises and remote.
* No noticeable Windows Updates relating to Exchange recently installed, though issue presented AFTER a server reboot.
* No issues logging in via OWA.

 

Things I've tried:
* Deleting Outlook Profiles and recreating.
* Removed stored credentials in Credentials Manager.
* Adding ExcludeExplicitO365Endpoint=1 in HKCU\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover.
* Changed Outlook Anywhere authentication method to NTLM from Negotiate and vice versa.
* Running testconnectivity.microsoft.com/tests/exchange tests (only receives minor warning about "can only validate the certificate chain using the Root Certificate Update functionality from Windows Update.").

 

Temporary Workaround:
* If the user closes the box, the prompt "Needs Password" appears in the Outlook taskbar, clicking that usually connects to Microsoft Exchange, but will prompt for the credentials again in the not too distant future.
* Instead of cancelling the dialogue box, simply dragging it off screen to prevent annoyance will keep the connection to Exchange open "forever".

 

For my and my 60 odd users' sanity, what can I check next please?

 

*EDIT* Panic over, I should know better than to not try "PLAN A - turn it off and on again" first... IISreset fixed the issue. :facepalm:

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