Here is one of the more helpful threads on sour crop. https://www.backyardchickens.com/forum/viewtopic.php?id=394635
It is possible that she has a different problem, so separating her from the others allows you to monitor her symptoms more closely and figure out what exactly is wrong.
The...
Poor hen. It sounds like she could have an impacted or sour crop, at least that is the first thing that comes to mind. If you have a dog kennel or even a large card board box, you set her up in there in the garage or spare room to keep an eye on her symptoms. Try putting a heat lamp over her...