I recently installed the duplexer option on my phaser 6120 and I have been getting the following error every time I print:
PAPER JAM MEDIASIZE (see below)
I can fix the problem every time by opening the duplex cover and then closing it. There is never any paper that actually gets jammed, and after it resets from the duplex cover being open an closed, it prints properly. I am using 8.5 x 11 standard US letter laserjet paper, and it printed fine before I added the duplex unit. I have the also made sure that the paper size in the default print preferences is "Letter".