Looking around I know this is a common error but I can't find for help for the specific issue I am having.
I am getting the error but in the message it is displaying the web part with it's assemblyFullName$ twice. 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.
I have verified that it is in the Safe Control Entries, it is in the Web.Config, and it is getting copied to the GAC with the name of UI_External. The PublicKeyToken and version number match everywhere. I've tried manually updating through the Safe Control Entries to have the assemblyFullName twice to try and match what the error is apparently looking for and this does not help fix anything.