What happens if you choose a debugger?

If you have performance monitor running when this happens, do you see if the w3wp service takes a lot of memory, cpu or something like that?

That could be anything.. What you do in your start page? Any stackoverflows, asking for content reference of the same content reference? Null reference exceptions.. What about log files? Windows Event log store? You may find some traces there..

Thanks for your help guys, there must have been someting with my IIS that caused the problem, I used another copy of our development environment and it worked as it should. 

Thanks!
Oskar

EDIT: A collegue have had the same problem and told me it could occur if Visual Studio 2015 beta had been installed. He had uninstalled it and got it to work again so if anyone experiences the same issue it might help to uninstall VS 2015 Beta. I had it installed on my machine so that might have been the problem for me as well.