Outlook keeps prompting for credentials after exchange CU21 upgrade

Question

Outlook keeps prompting for credentials after exchange CU21 upgrade. Could anyone help?
We have exchange 2016 and outlook 2016
The issue is happening only to a few mailboxes in a domain (we have multiple domains)

Tried the following
Reconfigured outlook profile.
Deleted the old xml file
No saved credentials on credentials manager
The 'Download Shared Folders' option is already switched off.
No additional mailboxes

The issue is happening only to the internal users. Can access ecp, owa without any issues
Its happening on cached and online mode.

ExternalClientAuthenticationMethod : Basic
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods : {Basic, Ntlm, Negotiate}

I also tried adding the following reg keys.but no luck

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover]
"ExcludeHttpsRootDomain"=dword:00000001
"ExcludeScpLookup"=dword:00000001
"ExcludeExplicitO365Endpoint"=dword:00000001

Answer 1

this is resolved. its a DNS issue :) Thanks all for your help

Answer 2

Two steps usually helps to resolve the issue.

1. Remove Negotiation in authentication method for EWS website in IIS and hit an IIS reset
2. Reboot the workstations affected and try again
3. If that doesn't help, here is a fix explained and try that in the workstations: outlook-not-connect-exchange-use-mapi-over-http