Configuration:
Ultimately, a Repair of the VS2012 resolved this issue for me. I followed the advice found at your social.msdn cross-post without any resolution (Devenv.exe /SafeMode /ResetSettings /ResetSkipPkgs and /Setup). Like you, my solution (VS 2010 SP1) also has a web application (targeting .NET 3.5), and the startup project is set to a winforms app. The ASP.NET development server did not start, nor did the app I was trying to debug.
Note that this issue was also posted to Connect at this link (by you?). If anyone else sees this issue, the Connect folks are requesting running the Microsoft Visual Studio 2012 Feedback Tool to collect data. As I started the Repair process prior to finding the Connect issue, I did not and was not able to provide feedback to MS with logging.