Syncronizing remote repositories on Satellite 6 fails with "Yum Metadata: Not a gzipped file"

Issue

• I want to setup a sync with epel.
• redhat syncs works fine but an extra repo sync with epel fails.
• Maybe I am doing it wrong, but the connection to the internet works via a proxy/firewall.

• This is what did: (I downloaded the GPG key with links, using the same proxy/firewall)

  hammer> gpg create --key /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7Server  --name 'GPG-EPEL-7server' --organization "<someorg>"
  GPG Key created
  hammer> product create --name='EPEL 3rd Party Packages' --organization "<someorg>" --description 'EPEL 3rd Party Packages'
  Product created
  hammer> repository create --name='EPEL 7 - x86_64' --organization "<someorg>" --product='EPEL 3rd Party Packages' --content-type='yum' --publish-via-http=true --url=http://dl.fedoraproject.org/pub/epel/7/x86_64/ --checksum-type=sha256 --gpg-key=GPG-EPEL-7server
  Repository created
  hammer> repository synchronize --organization "<someorg>" --product 'EPEL 3rd Party Packages'  --name  'EPEL 7 - x86_64'
  [......................................................................................................................................] [100%]
  Yum Metadata: Not a gzipped file
  

• Apparently there is some communication between our katello/sat6 and the "outsideworld."

• But the error makes no sense to me. Is there someting "dropped" by the proxy or is my config wrong?
• What does the error mean? How can I check all dependencies for this connection?
• Did I do something wrong or is there a mis(sing)configuration in de firewall/proxy config.