Old Agent Incremental Backup "<path-to-backup>" is not consistent: No Delta Backup Found, check Debug report
Some of the files belonging to one of the backup versions might be missing in the backup folder
The complete folder for that backup needs to be deleted (full backup folder including the subsequent incrementals) (i.e.: [path-to-version]).
Please note that this warning is regarding only that backup, all the following full backups should be consistent.