Comment 1 for bug 1747402

Are the volumes stored in a Ceph backup as well? Because the Ceph backup driver doesn't need the "--incremental" parameter to do incremental backups if the source volume is also stored in Ceph, and it doesn't do incremental for non Ceph volumes.

Where did you get that error? On the client? The API? Scheduler? Backup? Volume?

Could you include the traceback? Because I have just tested RBD incremental to incremental with devstack+ceph and it worked just fine with the CLI commands you mentioned.