This one has been bothering me for several weeks so I decided to research the problem and finally fix it.
The problem started when I downloaded a project from the web to start working on it. This same project worked previously, the zip was just an update.
Here’s what I found:
You may have already seen the article from Microsoft that says this is caused by trying to access a UNC path from your solution. But, my project doesn’t have any UNC paths in it.
Evidently, Microsoft is now adding a security property to any zip files that are downloaded from a security zone that is not trusted and applying that to your project file when the zip file is unzipped.
I eventually got this answer by visiting:
He has a utility to strip the information and a pretty good explanation of what is going on. But the real gem is in the comments where it says that all we have to do is right click the project file in File Explorer and click the “Unblock” button at the bottom.
Less than five minutes later I had my solution opening cleanly again.