Comment # 19 on bug 905034 from Johannes Meixner

A totally different idea:

Your office12-scanimage.open.strace shows:
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open("/home/per/.sane/hewlett-packard-scanjet-2400c.cal",...
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this together with what "man sane-genesys" tells about calibration
in particular things like
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  If you plug the scanner in another machine or use it with another
  account, calibration will have to be redone.
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and
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  --threshold percent
  0..100%  (in  steps of 1). Select minimum brightness to get a
  white point. Pixels whith brightness below that value will be
  scanned as black.
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and how your office12-scanimage-scan10.pnm looks like:

Perhaps your current calibration is wrong?

Could you move /home/per/.sane/hewlett-packard-scanjet-2400c.cal away
and test again and furthermore re-do the calibration and test again?