In fact the source is old NW behavior which dated back years in time (maybe 
from the very beginning?). I verified last year that it still exists. This is 
how NW works:

  - When you label a media, each good backup SW must read the label at the 
beginning to prevent accidental overwriting.
  - If it is a virgin tape, this will of course result in a read error. This is 
usually not a problem as it will be cleared later. Not when labeling a bunch of 
tapes ;-)
  - When you label more tapes than the "max. consecutive errors" (default: 20) 
the device will disabled (now put into service mode).

How can you solve the issue:
  - Increase the "Max. consecutive errors" value. But this could be problematic 
whenever a true problem will occur.
  - Do not label more than 20 tapes with one job.
  - Use auto media management (AMM) for the jukebox.

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