Symptoms
When you try to access a page that has third-party web parts or controls, the page isn't displayed successfully or some web parts show errors. When this issue occurs, the following error message and "6d3sf" or "apmev" event tags are logged in the SharePoint Unified Logging System (ULS) logs:
Missed type in new allowlist. Type = <blockedTypeName>
Cause
To strengthen its security, SharePoint has updated SPSerialization by adding a new allow list mechanism.
Resolution
Notes:
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Make sure that the content of the blocked types is safe before you follow the instructions to unblock it.
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If you have to include a backtick in the blockedTypeName parameter, you can double-enter the mark (``) to make sure that it's interpreted correctly. The blockedTypeName value is the generic type name instead of the assembly-qualified name.
To fix this issue, farm administrators can run the following PowerShell commands to add the blocked types to SPSerializationCustomizedAllowList:
add-pssnapin * $f = get-spfarm $f.AddGenericAllowedListValue("SPSerializationCustomizedAllowList", "<blockedTypeName>") $f.update() iisreset
