The primary reason for tangled strings is when the claw hits a prize on a weird angle and some but not all of the weight of the claw is taken up on the prize. The stop down switch does not activate and the motor keeps winding down causing more string to wind down than should. When the claw finally comes to a stop the stop down switch activates, claw grabs and it starts winding up. The slack string wont necessary be inside the up down winding spool - it can then go off the side of the pulleys winding round the axles.

Firstly get some slack in the string

To get slack in the string wind the up down motor pinion with your finger until there is enough slack

Then pull string off the front of the left right unit

Rewind the string N for normal

Check the UD fuse on the gantry fuse board - if blown replace with a 20mm 3A fuse

Let the claw hang down with a taut string when string is untangled - if too much string is out hold it yourself so that it is not slack

Turn machine on so it winds up by itself

NB It may be easier for you to remove the complete gantry to do this