Users in my environment have been experiencing an intermittent issue where they would periodically receive the following error when trying to access OWA:
“The custom error module does not recognize this error.”

Refreshing the page would resolve the issue so it wasn’t a critical problem but that made it no less annoying. As such, I turned to my good friend Google to see if he could be of any assistance.

After digging through pages of unrelated and unhelpful pages, I finally came across a page on Experts-Exchange that pointed me in the direction of something that seemed promising.

Like many of you, I’m not a huge fan of EE, especially not since the emergence of Server Fault and the rest of the Stack Exchange family. However, once in a blue moon it is actually helpful and this was one of those times.

I followed the link on that page to a post on Microsoft Technet. It was here that I found the fix (the information below has been changed to be more clear to follow and reflects some changes between Exchange 2007 and 2010):

1. Ensure that the “/owa” directory is running in the OWA Application Pool

• Launch IIS Manager
  • Start > Administrative Tools > Internet Information Services (IIS) Manager
• Expand the Server
• Expand Web Sites
• Expand Default Web Site
• Right-click the “owa” virtual directory > Manage Application > Advanced Settings
• Ensure that the Application Pool is set to “MSExchangeOWAAppPool”

This piece was already setup properly in my environment so I moved on to the next step.

2. Ensure that the MSExchangeOWAAppPool is running as Local System

• Launch IIS Manager
  • Start > Administrative Tools > Internet Information Services (IIS) Manager
• Expand the Server
• Expand Application Pools
• Under Name find “MSExchangeOWAAppPool”  and ensure that the Identity is set to “Local System”

Again, this part was already setup correctly in my environment, so I moved onto the final set of instructions:

3. Check the permissions for the ApplicationHost.config file and ensure the user ServerName\IIS_IUSRS has Read permissions for the file

• In Windows Explorer, navigate to c:\Windows\system32\inetsrv\config
• Right-click the ApplicationHost.config file
• Select Properties
• Click the Security tab and then click Edit
• Click Add
• In the Enter the object names to select box type ServerName\IIS_IUSRS, click Check Names, and then click OK
  
  
  • Note: ServerNameis a placeholder and should be replaced with the name of the machine you’re working on
• By default the user will have Read and Read & Execute permissions, uncheck Read & Execute (leaving only Read selected) and then click OK
• In the ApplicationHost.config Properties dialog box click OK

After adding the Read permission to the ServerName\IIS_IUSRS user on the ApplicationHost.config file, the problem stopped occurring and the calm was once again restored to our Exchange 2010 environment.