The main problem seems to be:
[2014-05-22 11:14:03,330: WARNING/Worker-2] Both a keyring file and keyring data were specified; ignoring the keyring file.
I suspect the celery job gets passed both a (valid) keyring_filename and an empty keyring_data and chooses the empty keyring_data. Hence the failure to validate simplestreams' data.
The main problem seems to be:
[2014-05-22 11:14:03,330: WARNING/Worker-2] Both a keyring file and keyring data were specified; ignoring the keyring file.
I suspect the celery job gets passed both a (valid) keyring_filename and an empty keyring_data and chooses the empty keyring_data. Hence the failure to validate simplestreams' data.