Hi Stefan,
according to the Netbackup Commands Guide for Unix, use the following command:
bpexpdate -recalculate -d 0 -force -client <clientname>
This command will set a new expirationdate to all backup images of client <clientname>. In this special case it sets the expiration to 0, which was in the past and therfore immediatly deletes the image.
Befor doing such a mass delete, I would do a fresh backup of the catalog.
Hope this works for you.
By
Christoph