Enstrom belt drive.
Having another stab at your problem ... as the blades are turning with the system disengaged, you have the belt too tight. The crankshaft drive pulley should turn easily inside the belt when the system is disengaged with the belt actually able to flop around slightly.
A second possibility is the drive side of the belt has become 'sticky' with ageing. A quick cure would be a good dose of French Chalk, but the overall problem is too tight adjustment.
PS. From inspection are all the belt grooves in good condition with no detached tearing on the drive face. How old is the belt?
Regards & good luck. Dennis K