I don't know how to cure it but I do know how to prevent it. You must provide your chickens with a good supply of "insoluble grit". There are two types "soluble" and "insoluble". Insoluble grit help a chicken grind up fiberous foods, like grass, lettuce and other things. ACV will not cure sour crop.
When folks feed their chicken lots of treats like bread and corn and grains chickens need the insoluble grit to help grind these up in the gizzard. Be sure to keep a big dish of this around even if they are ranged.
I wish you the best
Rancher