MIM portal error "Unable to process your request" after applying SharePoint Server updates

Symptoms

After you install one of the following SharePoint Server updates, Microsoft Identity Manager (MIM) portal pages aren't displayed:

• Description of the security update for SharePoint Server 2019: September 8, 2020 (KB 4484505)

• September 21, 2020, update for SharePoint Server 2019 (KB 4461512)

• Description of the security update for SharePoint Enterprise Server 2016: September 8, 2020 (KB 4484506)

• Description of the security update for SharePoint Foundation 2013: September 8, 2020 (KB 4484525)

• Description of the security update for SharePoint Foundation 2010: September 8, 2020 (KB 4486667)

See Ghosted pages in SharePoint don't render for more information. 

Cause

The security level of SharePoint ghosted pages was enhanced by forbidding server-side scripts and unsafe controls. Because MIM portal uses server-side scripts, its pages are blocked from rendering.

Workaround

To work around this issue until the next MIM hotfix is released, add a new PageParserPath element that has a VirtualPath attribute to the Web.config file under the configuration/SharePoint/SafeMode/PageParserPaths element, as follows:

     <SafeMode MaxControls="200" CallStack="false" DirectFileDependencies="10" TotalFileDependencies="250" AllowPageLevelTrace="false">
          <PageParserPaths>
               <PageParserPath VirtualPath="/identitymanagement/*" CompilationMode="Always" AllowServerSideScript="true" IncludeSubFolders="True"/>
          </PageParserPaths>
     </SafeMode>

References

Microsoft Identity Manager release history

Learn about the terminology that Microsoft uses to describe software updates.