Try this linke below.
Or i cut and paste text, but you should view pictures. If you cannot get to it from link. Try typing "Paper jams alternating between front and back door" It should be first link from HP about a 3550.
HP Color LaserJet 3500, 3550, and 3700 Series Printers - Paper Jams Alternating Between Front and Back Door
Customers may see paper jams error
messages with no paper seen in the paper path. The message will
alternate between check the front door and the back door.
The release lever engagement tabs on the
rear door can be bumped into the in-correct position. The purpose for
this tab is to re-engage the fuser release levers as the rear door is
closed. When one or both of these are left in the in-correct position
the printer goes into a cycle of alternating between a "Jam in the
Front Door" message and "Jam in the Rear Door" message. There is no
paper present in the paper path when this happens. When a customer gets
the "Jam in the Front Door" message and opens the front door, no paper
is found. After the customer closes the front door, they get the "Jam
in the Rear Door" message. When they open the rear door, find not paper
(even pull out the fuser), and close the rear door, they get the "Jam
in the Front Door" message. This cycle will continue
Placing both engagement tabs in the "Correct" position solves the problem and allows the print job to continue.for as long as the
customer is willing to keep checking.