I tried all of the above soloutions. none of them worked. I figured out that I had 2 keychains. Both of them had certificates and keys. I´ve deleted one keychain and removed all its key and certificates and just kept the "login" keychain. I´ve also removed everything from this keychain and then redownloaded the certificate from the apple developer page. After doing so everything was imported just into this ONE keychain. Cleaned Xcode, started. no problems!