Sounds like sour crop to me too... Massage the crop and get any gunk out that you can, add some cider vinegar to their water and restrict the diet to simple pellets for a few days. Its usually caused by a combination of long grass and grains which some chickens find hard to digest. That and of course overindulgence LOL.
Sourcrop is curable and you should find a lot of info and ideas to try. Its very rare that it gets so serious that it requires an operation and can usually be sorted out easily.