it's hard to tell but you might be able to change the angle of the lead of the halyard to the drum. This was always an issue with the first generation roller furling systems. These old systems rolled the sail around the forestay cable. If the lead was parallel to the stay (as yours is), the drum would spin with the forestay and roll the halyard around the stay. Once the halyard was spun up tight the drum would start to turn. The rub came when you wanted to lower the sail and the halyard was wrapped around stay. That and the chafe on the last foot or so of the halyard. To solve the problem was simple. At the appropriate spot, usually just above the drum, a halyard retainer was mounted on the mast. It's essentially a large eye strap and it would change the angle of the halyards approach to the drum so it couldn't spin. If you have the room, you could do the same thing and open up the angle and increase the space for your shackle and knot combination. You could place the eye a few inches down where the halyard exits the mast or anywhere that solved your issue. The strap is most likely cheaper then a new shackle as an extra bonus and it's an easy fix.