To answer my own question, in the keychain double click each certificate opens a new window with their properties. I opened all the certificates looking for the serial number mensioned in the error message. I found the certificate in question and deleted it from the keychain. Another attempt in xcode to validate the app and got the same error message but now with a different serial number of the certificate. I deleted that too from keychain. Another attempt and the validation of my screensaver got through.