You are probably experiencing a known issue in mender-artifact (see a few comments above). Try to update your mender-artifact tool to 3.1.0 (should be just a drop-in binary replacement): https://docs.mender.io/2.1/downloads
I updated the latest version of mender-artifact (3.1.0-dirty) by downloading and replacing the bin, but nothing to do, it still doesn’t work unfortunately:
Signature: signed; verification using provided key failed