Hey cschaf-
I just want to make sure I understand everything that is happening. Does Visual Studio open when you create a new code project or does that give you this error as well? Are you opening Visual Studio by double clicking on the .sln file? Are you able to open the VS solution for the project with File->Open Visual Studio through the editor? Are you able to open the solution through File->Open->Porject/Solution inside Visual Studio? It is also possible that the solution is being confused by the other Visual Studio versions. If possible you may want to remove them and try again.
Cheers