Just FYI, I just went through the same exact thing. Out of no where, I could not get past "undefined paper".
Sloution for me was SIMPLE, the Y-sensor was bad(probably very dirty). $55 or so from GEI Calgraph (and that was overnight service) and it now works perfectly.
My opinion, your Y paper sensor is either bad, or very dirty....this should be an easy fix.
Id just get a new sensor...it cant hurt to have a new one, and if that doesnt work, you eliminate that as a possibility period.
Good Luck! James