Issue when Webpart with a custom toolbar is loaded after SharePoint 2019 September 2024 CU installation

Question

Hi,

After installing the SharePoint 2019 September 10th Cumulative Update (this one), our custom webpart toolbar property appears to be causing an issue when attempting to render the webpart control. The exception we encounter is the following:

[ArgumentException: The serialized data is invalid.]
  at Microsoft.SharePoint.WebPartPages.SPObjectStateFormatter.Deserialize(Stream inputStream)
  at Microsoft.SharePoint.WebPartPages.Utility.DeserializeByteArrayToObject(SPSerializationBinderBase binder, Byte[] bytes)
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.LoadInitialWebPart()
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.Deserialize()
  at Microsoft.SharePoint.WebPartPages.SPWebPartManager.CreateWebPartsFromRowSetData(Boolean onlyInitializeClosedWebParts)

[UnsafeControlException: The control type 'System.Collections.Generic.List`1[[IC.He.SharePointWeb.SP.WebParts.WebpartColumn, IC.He.SharePointWeb, Version=1.0.0.0, Culture=neutral, PublicKeyToken=50a0670ec03a0037]]' is not allowed on this page. ]
  at Microsoft.SharePoint.WebPartPages.SPSerializationBinder.IsAllowedType(Type type)
  at Microsoft.SharePoint.WebPartPages.SPSerializationBinderBase.CheckIfAllowedType(Type type)
  at Microsoft.SharePoint.WebPartPages.SPSerializationBinderBase.BindToType(String assemblyName, String typeName)
  at System.Runtime.Serialization.Formatters.Binary.ObjectReader.Bind(String assemblyString, String typeString)
  at System.Runtime.Serialization.Formatters.Binary.ObjectReader.GetType(BinaryAssemblyInfo assemblyInfo, String name)
  at System.Runtime.Serialization.Formatters.Binary.ObjectMap..ctor(String objectName, String[] memberNames, BinaryTypeEnum[] binaryTypeEnumA, Object[] typeInformationA, Int32[] memberAssemIds, ObjectReader objectReader, Int32 objectId, BinaryAssemblyInfo assemblyInfo, SizedArray assemIdToAssemblyTable)
  at System.Runtime.Serialization.Formatters.Binary.__BinaryParser.ReadObjectWithMapTyped(BinaryObjectWithMapTyped record)
  at System.Runtime.Serialization.Formatters.Binary.__BinaryParser.Run()
  at System.Runtime.Serialization.Formatters.Binary.ObjectReader.Deserialize(HeaderHandler handler, __BinaryParser serParser, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
  at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream serializationStream, HeaderHandler handler, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
  at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream serializationStream)
  at Microsoft.SharePoint.WebPartPages.SPObjectStateFormatter.DeserializeValue(SerializerBinaryReader reader)
  at Microsoft.SharePoint.WebPartPages.SPObjectStateFormatter.DeserializeValue(SerializerBinaryReader reader)
  at Microsoft.SharePoint.WebPartPages.SPObjectStateFormatter.Deserialize(Stream inputStream)

In our web.config we have defined our assembly as a safe control in the following manner:

      <SafeControl Assembly="IC.He.SharePointWeb, Version=1.0.0.0, Culture=neutral, PublicKeyToken=50a0670ec03a0037" Namespace="IC.He.SharePointWeb.SP.WebParts" TypeName="*" Safe="True" />
      <SafeControl Assembly="IC.He.SharePointWeb, Version=1.0.0.0, Culture=neutral, PublicKeyToken=50a0670ec03a0037" Namespace="IC.He.SharePointWeb.WebParts" TypeName="*" Safe="True" />

I have tried tinkering with the AllowListItems, since it seems to be related to the serialization of collections, but I couldn't find a value that corresponds to the Sytem.Collections.Generic.List type.

I attach the full correlation log too: webpart-log.txt

Any help would be appreciated