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Fixing SharePoint Designer Errors July 2026 CU

After installing SharePoint Subscription Edition July 2026 CU, there is again error in SharePoint Desgner when creating custom forms.

Errors in ULS logs 

07. 15. 2026 10:35:57.74	w3wp.exe (0x1600)	0x14DC	SharePoint Foundation	General	377si	Unexpected	Re-definition of tag <SharePoint:FieldDescription> found, maintaining consistency of attributes across duplicate tags for security. To bypass validation, disable farm property 'PreserveAttributeConsistency'.	7f7127a2-b201-706a-d161-b289d54d2ec3
07. 15. 2026 10:35:57.74	w3wp.exe (0x1600)	0x14DC	SharePoint Foundation	General	377si	Unexpected	Re-definition of tag <SharePoint:FormField> found, maintaining consistency of attributes across duplicate tags for security. To bypass validation, disable farm property 'PreserveAttributeConsistency'.	7f7127a2-b201-706a-d161-b289d54d2ec3
07. 15. 2026 10:35:57.74	w3wp.exe (0x1600)	0x14DC	SharePoint Foundation	General	5mh3d	Medium	Unsafe control=Microsoft.SharePoint.WebPartPages.DataFormParameter, Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c for having property traversal char in attribute value.	7f7127a2-b201-706a-d161-b289d54d2ec3
07. 15. 2026 10:35:57.74	w3wp.exe (0x1600)	0x14DC	SharePoint Foundation	General	ajlz0	High	Getting Error Message for Exception Microsoft.SharePoint.WebPartPages.WebPartPageUserException: Uvožena datoteka ni veljavna. Preverite, ali obstaja opisna datoteka spletnih gradnikov (*.webpart ali *.dwp) in ali vsebuje ustrezno zapisano kodo XML.     at Microsoft.SharePoint.WebPartPages.WebPartImporter.CreateXdoc(XmlReader reader)     at Microsoft.SharePoint.WebPartPages.WebPartImporter..ctor(SPWebPartManager manager, XmlReader reader, Uri webPartPageUri, SPWeb spWeb)     at Microsoft.SharePoint.WebPartPages.WebPartImporter.Import(SPWebPartManager manager, XmlReader reader, Boolean clearConnections, Uri webPartPageUri, SPWeb spWeb)     at Microsoft.SharePoint.WebPartPages.ToolPane.GetPartPreviewAndPropertiesFromMarkup(Uri pageUri, String webPartMarkup, Boolean clearConnections, SPWebPartManager manager, SPWeb web, MarkupOption markupOption, Boolean bConvertWebPartFormatBehavior, Boolean prependRegisterDirectivesToMarkup, WebPart& frontPagePart, String& markupStorageKey, String& frontPageZoneId, WebPartImporter& webPartImporter, List`1& registerDirectiveDataList, IServerDocumentDesigner& documentDesigner, Boolean blockPropertyTraversal)	7f7127a2-b201-706a-d161-b289d54d2ec3
07. 15. 2026 10:35:57.74	w3wp.exe (0x1600)	0x14DC	SharePoint Foundation	General	avfx	Medium	SOAP exception: Microsoft.SharePoint.WebPartPages.WebPartPageUserException: Uvožena datoteka ni veljavna. Preverite, ali obstaja opisna datoteka spletnih gradnikov (*.webpart ali *.dwp) in ali vsebuje ustrezno zapisano kodo XML.     at Microsoft.SharePoint.SoapServer.WebPartPagesWebService.ConvertWebPartFormat(String inputFormat, FormatConversionOption formatConversionOption)	7f7127a2-b201-706a-d161-b289d54d2ec3

Main error that appears is related to unsafe control

Unsafe control=Microsoft.SharePoint.WebPartPages.DataFormParameter, Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c for having property traversal char in attribute value

Resolution to the error is to add a web.config entry

<SafeControl Assembly="Microsoft.SharePoint, Version=16.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" Namespace="Microsoft.SharePoint.WebPartPages" TypeName="DataFormParameter" Safe="True" 
    AllowPropertiesTraversal="True" />

Hope this helps

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