Stripped locknut on chain adjuster.

Spray the adjuster with a penetrating oil and let it sit awhile. Then try using a pair of vise grips on the adjuster locknut, and a socket on the adjuster and try to loosen both at the same time. If it's still frozen, use a torch to heat the area around the adjuster and tap on the adjuster to break it free. Since the adjuster is steel and the swingarm is aluminum, lubricate them frequently to reduce the chemical reaction and corrosion between the two.

i see you fixed it already but for others having this problem try removing the axle and axle blocks and using a deep six point socket on the nut