The DEFAULT and FUTURE levels of crypto policies don't allow AEAD ciphersuites in gnutls.
crypto-policies-20150305-1.gitf618101.fc21 has been submitted as an update for Fedora 21. https://admin.fedoraproject.org/updates/crypto-policies-20150305-1.gitf618101.fc21
Package crypto-policies-20150305-1.gitf618101.fc21: * should fix your issue, * was pushed to the Fedora 21 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing crypto-policies-20150305-1.gitf618101.fc21' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2015-3324/crypto-policies-20150305-1.gitf618101.fc21 then log in and leave karma (feedback).
crypto-policies-20150305-1.gitf618101.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.