I have found the solution to this or a similar problem if you are starting Visual Studio by loading a solution file.
The symptoms are that a solution that previously could be loaded with no problems, suddenly starts having the unknown error box appear for every project in the solution. Sometimes you can't see the relationship to the continuous reappearance of the error message and the project loading, because you don't have the solution explorer open, so you aren't even aware there is a correlation.
The solution to this is to use task manager to stop visual studio, confirm with visual studio that you wish to shut it down and then look for the .suo file for the solution you are trying to load. In VS2012 it appears in the same directory as the solution file and has the extension solutionname.v11.suo while in VS2015 it appears in a hidden .vs directory under the location where the solution file is found, in a folder named for the solution.
Delete this file and the problem will go away.
The .suo file contains the user preferences for the solution you are trying to load and it is perfectly safe to delete it as it is regenerated when you load the solution.
If you are getting the unknown error when you haven't tried to load any solutions, this is probably not the right answer.