Unable to find a document class... com.apple.InterfaceBuilder3.CocoaTouch.XIB

I have two versions of Xcode installed: Xcode 4 in /Developer/ and Xcode 3 in /User/Xcode3/.

While working on an iPhone app in Xcode 4 I suddenly got this error:

I have no idea where or why this has started. I thought it might be related to this post, but the main solution which has received 32 up votes does not work for m开发者_C百科e. I did notice that my Xcode 4 folder was missing the InterfaceBuilder app for some reason which I then copied and pasted from my Xcode 3 folder. Not that it made any difference.

Any ideas?

EDIT 1

Also, I should note that even though when I 'get info' on the project and it states that it defaults open to Xcode 4 when I double click on the project it opens with Xcode 3. Therefore I choose to open with Xcode 4. Still doesn't help.

EDIT 2

Also, when I click on the .xib files, I just see string values, no GUI.

I completely reinstalled Xcode 4 and that seemed to do the trick. But I must say I am quite paranoid when I open up projects. I have the impression that if I use the wrong version of Xcode with the wrong project this problem will come back.

Any illumination on this bug would be greatly appreciated.