A couple things you can try:
- tighten the other thumbscrew (on the other side) then unscrew each one evenly. i.e. give a few turns to the left one, then the same amount to the right one. They get stuck if you unscrew one all the way, then the other, as the threads on the one that's still in bind.
- the other thing that may have happened is that the little hex nut that the thumbscrew screws into (the hex nut to one side of the serial port) has come loose and is turning freely. You need to grab it with a pair of fine needlenose pliers so it doesn't turn, then you will be able to get the thumbscrew out. Next thing you need to check is that the nut on the inside of the Amiga casing that held the outer hex-nut into place hasn't fallen off (which is usually why these things start turning freely). If that nut is rattling around inside your Amiga, it could cause a short circuit.
- lastly, it is possible the plastic "handle" on the thread screw has come loose from the actual shaft of the screw. In this case I'd break it off with pliers and grab the remains of the metal shaft with small vise-grips in order to turn it - then get a new serial cable.
