Symptoms

When you try to sign in to Outlook on the web or the EAC in Exchange Server, the web browser freezes or reports that the redirect limit was reached. Additionally, Event 1003 is logged in the event viewer. For example, the following entry is logged:

Event ID: 1003
Source: MSExchange Front End HTTPS Proxy
[Owa] An internal server error occurred. The unhandled exception was: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.HttpProxy.FbaModule.ParseCadataCookies(HttpApplication httpApplication)

This issue occurs if the Exchange Server Open Authentication (OAuth) certificate is expired, not present, or not configured correctly.

Resolution

To Resolve this problem carry out the following:

1. Open Exchange Management Shell as Administrator
2. Run the following command. (Replace contoso.com with your SMTP domain)

New-ExchangeCertificate -KeySize 2048 -PrivateKeyExportable $true -SubjectName "cn=Microsoft Exchange Server Auth Certificate" -FriendlyName "Microsoft Exchange Server Auth Certificate" -DomainName "contoso.com"

3. Take note of your thumbprint, you’ll need it for the next command. Now run the rest of the commands.

Set-AuthConfig -NewCertificateThumbprint <ThumbprintFromStep1> -NewCertificateEffectiveDate (Get-Date)
Set-AuthConfig -PublishCertificate
Set-AuthConfig -ClearPreviousCertificate
Restart-Service MSExchangeServiceHost
Restart-WebAppPool MSExchangeOWAAppPool
Restart-WebAppPool MSExchangeECPAppPool

4. If you have multiple Exchange servers, you’ll need to run the following commands on each of them, but wait for the new Exchange Auth Certificate to be replicated to them first.

Restart-Service MSExchangeServiceHost
Restart-WebAppPool MSExchangeOWAAppPool
Restart-WebAppPool MSExchangeECPAppPool

 Wait, this can take a few hours-48hours to replicate across (more than the one hour Microsoft state), but then everything will start working again. If you wish to confirm each server is aware of the new Auth configuration you can run “Get-AuthConfig” and validate the Thumbprint and effective date match your new certificate and the time you executed the first “Set-AuthConfig” command respectively. If you have a Hybrid Exchange environment you need to rerun the “Hybrid Configuration Wizard” again to update these changes to Azure Active Directory.

(Get-AuthConfig).CurrentCertificateThumbprint | Get-ExchangeCertificate | Format-List

Good luck

Link Reference: https://docs.microsoft.com/en-us/exchange/troubleshoot/administration/cannot-access-owa-or-ecp-if-oauth-expired

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