I had a similar thing happen on the 60HC deck on my x758. The set screw holding the spline collar onto the shaft backed out and made it really hard to disengage the deck from the tractor. Looks like the OP had a worse case of the same type of thing. Glad they got it fixed for you. Check those set screws from time to time or pull them out and put some loctite on them.It looks to me as though the male spline collar has come loose from the gearbox shaft and is stuck inside the tractor pot connector.
On the gearbox shaft there should be a woodruff key that engages the spline collar and the collar is held on this shaft with two small metric set screws.
I have had these set screws come loose and protrude outside the collar and interfere with clean coupling.
Make sure you still have the woodruff key, reinstall the collar and use low strength loctite on the set screws to prevent this from recurring in the future.