It's possble that the crop is doughy.
I'd check it first thing like you did the other one.
When is the last time you wormed your flock?
Crop problems may be a combination of the "categories" detailed in the article below. For instance, a slow/doughy and/or impacted crop can turn sour. I always use coconut oil when treating a crop, but you may need to step up your treatment to include for doughy/impacted, etc.
Do keep in mind that crop issues are often a symptom of an underlying condition.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/