MY nice HP LaserJet 6L printer started to play tricks.  When it
prints, all the blank sheets come out with the printed page.  Probably
there is somebody who had the same problem.

This is a paper feed problem that usually happens to old printer. You
will need to unplug and open your printer and clean the feed roller
and with alcohol. Be careful not to touch any electronic parts.

Where I worked in the past, we had the same problems with a lot of HP
LJ6L. The technician told us that the paper roll were consumed.
Repairing the printers was too expensive, so the solution was...
insert a page at time :-D