VisualStudio 2010 Cannot load class diagram "Exception of type 'System.Exception' was thrown."

In a VisualStudio 2010 solution with many projects one of the projects contains some class diagrams. When I try to open them I get the error message `Cannot load '<class diagram file>': Exception of type 'System.Exception' was thrown.` Besides googling (I only found solutions for some different problems) I tried the following things: - removed some and all class diagram content: got the same error - added a new class diagram to the project: got the same error - created a new solution & project, and added a new class diagram: it worked! - created a new solution and added the existing project with the class diagrams: it worked! - deleted SUO and SDF file: got the same error Any idea how I can fix my solution? Creating a new solution would be possible, but tedious, and I would risk to encounter the same problem again. Because the solution is copy right protected and massive I cannot provide it.