Unfortunately this approach didn't work out for me. But here's a fix which worked for me (to get this to work you need a working project file on Subversion or so):
I did roll back to an working version of my project file. As it isn't possible to revert with Xcode (Where is the 'Revert' option in Xcode 4's Source Control?) - I used Tortoise, my Windows machine and this Tutorial (http://tortoisesvn.net/docs/nightly/TortoiseSVN_en/tsvn-howto-rollback.html) to roll back to an older project file.
As the Tutorial didn't work out for me, I just used Tortoise to save the working revision of my project file to an usb stick to port it to my mac. After that I replaced the new broken project file with the old working one, cleaned and it worked like a charm!