error connecting to undo manager of source file
The first fix in my case was to close all instances of Visual Studio, re-open them, recompile and run my application. This fixed the error in my case. I have also read that making a change, saving it, undoing the change and then saving it again will also fix the error. In my case this was a Winforms application that was calling a C# class library (the file in question being in the C# library).
This did not work for me but has been reported to work for others. Make a change to the file in question, save it, then undo the change and save it again.
Scott Hanselman blogged back in 2007 that he was able to exclude the file in question from his project, compile, then include the file and recompile and that fixed the error. Here is a link to his entry on that subject: