Oh god...

That's the worst failure I've seen of a printer (no fault of yours!) I have had a twist halfway through a spool before, but because of the way the flashforge is designed, if this happens, yes the roll goes tight but eventually the gear just eats through the plastic and then it snaps when theres not much left... but to actually remove the entire gantry, holy cow!!!!!

"Just check that the filament can feed freely before printing" is shit advice from them, because you can't see beyond one or two layers into a spool!
No, this is the absolute best advice you can give to anyone with any printer. You LOAD FIRST before unloading. I don't know why it's not up in lights flashing on every printers website!! ALL my printers, FFs.. kossel, flashforge, prusa.. printrbot.. load first. Clear the shit out, push the crap through the nozzle, not pull it back up through the gears where its cold - if you do this you pretty much never get blockages.