The … package did not load correctly
This issue has happened to me repeatedly on Visual Studio 2019 and the package concerned has varied considerably, it appears to be related to when I uninstall extensions that it goes particularly bad. Sometimes when you open Visual Studio you get a number of these messages and the option to ignore the message in future doesn’t prevent it from recurring.
The solution to this is remarkably straightforward, firstly shutdown all instances of Visual Studio, then delete the ComponentModelCache folder which can be found in the following folder
the user name is your login name. You may find in the VisualStudio folder a number of folders that are 16.xxx — I just deleted the ComponentModelCache folder from the one that had been used most recently and all seemed fine.