Some functions on customized SharePoint pages don't work (KB5000640)

Symptoms

Some functions don't work on customized SharePoint pages that use the SPWorkflowDataSource or FabricWorkflowInstanceProvider user control, and you see entries for the 9zu4p or 9zu4q event tag in SharePoint Unified Logging System (ULS) logs. This problem occurs after you install the following February 9, 2021, SharePoint security updates:

Cause

The security rule is enhanced when you convert types in SPWorkflowDataSource and FabricWorkflowInstanceProvider. Only trusted types can be converted.

Resolution

To restore the inoperative functions, add the blocked types as trusted types in the Web.config file if the blocked types are safe.

To do this, add a new AllowConvertType element that has a type attribute to the Web.config file under the configuration/SharePoint/SafeMode/AllowConvertTypes element, as follows:

<SafeMode MaxControls="200" CallStack="false" DirectFileDependencies="15" TotalFileDependencies="250" AllowPageLevelTrace="false">
<AllowConvertTypes>
<AllowConvertType type="<allowedConvertTypeName>, <allowedConvertTypeAssemblyName>, <AssemblyVersion>, <AssemblyLanguageSetting>, <AssemblyPublicKey>"/>
</AllowConvertTypes>
</SafeMode>

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