Hi Frans
Just thinking out loud with some of this this have you tried turning it by hand with the clutch pedal down

. Was wondering if somethings happened to the input shaft bearings or something there and locked the motor, because it can't turn the shaft into the gearbox, could be the clutch cover and something happened and jammed in there to maybe

.
I guess you have tried turning the motor back and forward by hand, as a small amount of water in a cylinder, will lock it from going over compression but you should be able to turn it back, and the same with a dropped valve or something like that. You say the starter turns, have you taken the motor off , as i have seen starters shear the nose from the end and the part jam the flywheel.
It could be a seized piston or bearings, but I would of figured for a bearing you might of seen the oil light flick or seen a drop on the pressure gauge before you stopped it

. Timing gears might have lost a tooth, or even the injector pump seizing could make it lock up I guess to

. Not being there and seeing the tractor kinda leaves us guessing a lot with this sadly

.
Seems odd to me and hope it's not a bearings, crank and oil pump

.
Regards Robert
A Fordson is for life not just for Christmas !.