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What does it mean to have a web part error?
I.E. Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type External.UI_External.UI_External, External, Version= [redacted], Culture=neutral, PublicKeyToken= [redacted] could not be found or it is not registered as safe.
Is the class name of your web part correct?
The class name of your Web part is UI_External (thus its full name is External.UI_External.UI_External: is it correct, or is there also an error here? I strongly suggest you to carrefully review the names of your project/assembly/namespaces/classes and be sure they’re all set correctly.
How to search web part entry in web config file?
Try to search your web part entry in web config file. In cases, when you save site template from web application and try to use in different web application you can face this issue. Or sometimes, people download the .webpart file from web part gallery and upload them to different web application which results in Not a Safe Control issue.
How to search safe control entry in web config file?
The safe control entry looks like following: Try to search your web part entry in web config file. In cases, when you save site template from web application and try to use in different web application you can face this issue.
Description of the security update for SharePoint Enterprise Server 2013: September 8, 2020 (KB4484515) An allow list of ASP.NET controls is enforced in SharePoint 2010 and SharePoint 2013 after you install updates. If web parts or controls on the SharePoint pages are not added to the allow list, the pages do not render.
After you install one of the following updates, some SharePoint pages do not render, and generate error messages such as the following: Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or imported. The type could not be found or it is not registered as safe.