Saw the Video and posted a reply.
I believe that this is a long bearing screw, likely the lower right bearing. Back in the early days, the Truck Castings were thinner on the lower right bearing. The Wedge effect of the screw head would sometimes snap the truck. A LHR fix was to put a black washer on the screw and that worked GREAT... They may have used a longer screw too... Later Truck castings had a thicker lower right area plus they filled in the now un used holes like the probe data and homing sensor....
So that leads me to believe that a longer screw may have been re cycled and is now sticking out the back of the lower right bearing and hitting the lower frame.... A thin file or sand paper might let you sand off the long screw.... Getting the screw out with the lock tite can be a hard thing to do without damaging the screw head. Heat may be needed....
Good Luck,
AL
EDIT: AND.... LOOK at the Y Truck housing and the 2 holes where the Z Gearbox Attaches... IF you failed to put the Washer on the Z Gearbox Screw OR a longer Screw was used... It could stick out and snag the Z Truck....