Before Xcode 6, it was sufficient to provide only a *.p12 certificate and *.mobileprovision file to Xcode in order to export an *.ipa file for Ad Hoc builds.
We were having the same problem. I tried building the project and making the archive using Xcode 6 and after that I go to the Organizer window of XCode 5 and export the adhoc build from there. It works fine.