Thanks for sending the video. Yes, this is the lock nut we are using but the problem is NOT that we don’t know how to unlock it in the usual way, using a spanner. It is that for some reason the thread on the nut or screw seems to have failed so that it is permanently jammed and the spanner will not work.
Assuming that is the case, I guess we’ll have to saw through the screw in order to release the other components, including the wheel.
Have you had any problems with defective screw threads or nuts? - maybe we were just unlucky.