The message just means what it is all about: There is no complete and 
successful full backup found for this save set which can be taken as a 
reference for an incremental backup. Consequently, no incremental or level 
(differential) backup can be run. This is logical. 
Please read the manual about backup level and dependencies.

Why does it happen? - Only YOU know. But if you really see a full, complete 
backup with mminfo, then this should not happen. NW will report this info only 
because could not find a full in the media db. However, a media corruption 
problem will only occur during reads.

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