exchange 2019 Owa null reference error

Question

exchange 2019 Owa null reference error

Mohammed Rafeeq uddin 0

I am running exchange 2019 Cu14 with software Update 1 when access OWA it ask sign in authorization username and password after enter it got OWA null reference error and in event there are lot of event with event id 1309 Exception information:

Exception type: NullReferenceException

Exception message: Object reference not set to an instance of an object.

at Microsoft.Exchange.Clients.Owa2.Server.Core.OwaModule.ShouldBlockIncomingRequest(HttpContext httpContext)

at Microsoft.Exchange.Clients.Owa2.Server.Core.OwaModule.OnAuthenticateRequest(Object sender, EventArgs e)

at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()

at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)

at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

request information :

request url : exchange.domain.com:444/owa/favicon.ico

requested path /owa/favicon.ico

Chris-D 1,180 Reputation points Independent Advisor

2026-09-09T22:47:26.93+00:00

Hi Mohammed,

Have you had a chance to review the information provided? Any updates would be greatly appreciated.

1 answer

Your answer

Chris-D 1,180 Reputation points Independent Advisor

2026-09-09T22:47:26.93+00:00

Hi Mohammed,

Have you had a chance to review the information provided? Any updates would be greatly appreciated.

Answer 1

Hi Mohammed,

Thank you for sharing the detailed error information.

The request to /owa/favicon.ico is likely where the exception was recorded rather than the actual cause. The stack trace indicates that the OWA module is failing during the authentication stage on the Exchange Back End website. Since the error occurs after credentials are entered, it may be related to an incomplete Security Update installation or inconsistent OWA/IIS files and configuration.

Here are some steps you can try:

As these steps require administrative access and may affect Exchange services, they should be performed by an Exchange administrator during an appropriate maintenance window. Please ensure that a valid backup is available before proceeding.

1/ Confirm the installed Exchange build and update

Please run the following command in the Exchange Management Shell:

Get-ExchangeServer | Format-List Name,AdminDisplayVersion

Also check Control Panel > Programs and Features > View installed updates and provide the exact KB number of the latest Exchange Security Update.
Additionally, confirm whether the issue started immediately after installing CU14 or a Security Update.

2/ Determine the scope of the issue

Verify the following:

Does the issue affect all users or only one mailbox?
Can you access the Exchange Admin Center?
Are Outlook desktop connectivity and mail flow still working?
Does the same issue occur when accessing OWA directly from the Exchange server?
If your environment contains multiple Exchange servers, does the issue occur when connecting directly to each server?

3/ Run the Exchange Health Checker

Download the latest Exchange Server Health Checker script and run it from an Exchange Management Shell opened as Administrator:

.\HealthChecker.ps1

Review the generated HTML report for items highlighted in red or yellow and share the relevant findings after removing any sensitive information.
The Health Checker supports Exchange Server 2019 and must be run with the appropriate administrative permissions.

4/ If the issue started after installing a Security Update

If the Security Update was installed manually, consider reinstalling the same update from an elevated Command Prompt:

Confirm that the Security Update matches the installed Exchange CU.
Open Command Prompt by selecting Run as administrator.
Run the .msp installer using its full path.
Restart the Exchange server after the installation completes.

For example:

"C:\Updates\Exchange2019-KBxxxxxxx-x64-en.msp"

Please also share the Exchange build, installed Security Update KB, Health Checker findings, and whether ECP is also affected.

For additional information, you may find these Microsoft articles helpful:

I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.

I look forward to hearing your thoughts on this.

If the answer is helpful, please click 'Yes' and kindly upvote it.

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