Next to the message in the subject, one of the symptoms we encountered is that although the solution contains some test projects the ‘Create private accessor’ menu has disappeared. Also, selecting ‘Create unit tests…’ produces an error. And when editing a testrun config and selecting the ‘Code coverage’ option, the settings dialog simply closes, without any message.
All these (and probably more) symptoms can occur because of a corrupt solution file. In our case the nested projects section was corrupted. Chances are this is because of some project or solution folder not being removed the way it should have been. Anyway, I tried fixing the solution file by hand but because of too many projects and solution folders this wasn’t the way to go. Those GUIDs don’t make a file very readable… I made a new solution file, containing the same solution folder structure, and added the existing projects. I overwrote the existing solution file, and that was that. So when you’re experiencing any or all of the above symptoms, try building up a new solution file.
Hope this helps.