I had the same problem and finally managed to make it work.
First of all, I added my server to my developer team but I had the same error as you.
Then I copied my key from user keychain to system keychain as indicated here but it was still not enough.
And finally, I copied the provisioning profiles from ~/Library/MobileDevice/Provisioning\ Profiles/ to /Library/MobileDevice/Provisioning\ Profiles/ (in fact I copied all the MobileDevice directory because it didn’t exist).
And now, I have a working continuous integration giving me an ipa which I can download on my Mac or install over the air on my devices.
I don’t know why I had to do this, but I hope it will work for you too.