Well.. for me it was Telerik JustMock Q3 2013 (13.3.1015.0) that caused the problem. Uninstalled it from VS 2013 and the problem has gone..
see also ASP.NET-MVC4 Code Not Running and http://feedback.telerik.com/Project/105/Feedback/Details/63749-unable-to-debug-asp-net-projects-with-q3-2013
One lost day and many new white hairs... Curse on you Telerik guys! ;)