First, there's one old question with the same issue, but since the answers don't work for me, i'm posting this one:
I had problems to open an old project at all. So i deleted xcuserdata and the derived data, from the options tab, too. This one worked fine and i was able to open the project, but now when i click on the storyboard Xcode "hangs" and i have to force it to quit.
This happens only if i click on main.Storyboard, every other file in the project works fine when i click on it.
The last thing i tried was to run the project and in the last 5 minutes, it says Compiling 2 of 2 Storyboard files and that's all; no other result at the moment.
edit: cleaning the project doesn't work too.
edit2: It seems the problem is with my Xcode, not the project. I created a new project and Xcode freeze when i try to open the storyboard. Same with some other old projects. Next step: reinstalling the Xcode.