Impacted crop, digestive system and/or gizzard - maybe a blockage somewhere.
No, I wouldn't treat a blockage with bread and olive oil. I'd treat according to the article I linked previously. I've used coconut oil and stool softener to help get things moving when I suspected a blockage. A molasses flush may help if those 2 things don't work.
Here's the deal. A crop problem fall under several of those categories in the articles, blogs, etc. that you read. A crop can be sour, but still be impacted. There can be a blockage or partial blockage and the crop be doughy or sour...see what I mean. It can be a multi-layered issue.
Ime, a crop issue is usually a symptom of an underlying condition. Reproductive problems like cancer, internal laying, etc., worms, Coccidiosis and/or infection - these seem to be conditions that accompany a crop issue.