When I try to open any activity in design mode designer fails with message "The document contains errors that must be fixed before the designer can be loaded. Reload the designer after you have fixed the errors". The project builds and runs fine though. This happens even with a fresh newly created solution. I mean that if I create a new workflow project and add a new item -> activity the problem persists. If I click "Reload the designer" I get the following error: "Undefined CLR namespace. The 'clr-namespace' URI refers to a namespace 'System' that is not included in the assembly". If I build the project the error disappears but the designer doesn't start working.
I'm using VS2010 Premium. The only plugin installed is ReSharper. However I tried removing it but it didn't solve the problem. I've also asked a friend of mine to reproduce the same error on his computer but everything worked well.
The problem was that there are several types of editor for .xaml files. If you right click open with you can choose one of them. In my case WPF designer was selected by default though it's better to choose automatic selection.
Hope this information will be helpful for someone else :)