Backup will fail when using RBAC roles to perform backups because cbbackupmgr is hitting the REST endpoint to fetch the collection manifest from the cluster even though in 6.6.0 it doesn't support collections. This is done in a BTAFTP approach, however, this doesn't take into account RBAC.
When performing a backup with either the 'data_reader' or 'data_backup' role you will see an error similar to:
The Couchbase Source/Sink should use the cluster version number to determine if the cluster supports collections. If it doesn't then we shouldn't hit this endpoint at all.