I have an HP Color LaserJet 4600dn that has given me all sorts of problems with duplex jamming. It gets the 2nd page stuck in the Paper Pickup Assembly about an inch into it with the preceeding page stuck halfway through the fuser (on its way back INTO the printer to have its second side printed).
I followed this guide: http://www.lbrty.com/tech/tarticles/ta46xxJAM.pdf which helped me replace the RB2-8306-A Junction Guide Assembly. I've also swapped out the paper tray and tried swapping it with a known good feed belt, but neither of those did anything. Does anyone have any idea what might still be causing this jam? Is there a sensor problem that I'm missing somewhere?
When you guys say "paper feed assembly" do you mean what's also called the "Paper Pickup Assembly" part # RG5-7453 (or the thing I thought I fixed, ugh!)?
Here are some pictures of the stop condition when the error happens. You're right that the lead page is about 2 inches into the Paper Pickup Assembly (having gone past the end of the back-side of the transfer belt) when the error happens.