A grey rectangle appears on the scanned pages, as shown in the attachment.
It looks like only a video artifact, though, as the generated pdf is alright.

Thanks for the report. That grey block is not part of your scan and will
not be in the saved PDF or DjVu: it is scantpaper’s selection highlight,
drawn over the preview at 20% opacity. The rectangle is kept while you
move between pages so you can preview a crop on each page, and it is
never written into the output, so your PDF is correct.

It currently cannot be dismissed without restarting the application,
which I agree is not obvious.

I'm currently adding a menu item and Esc accelerator to improve the UX
for the next release.

Regards

Jeff