Yes, the crop can get "impacted" and then the food in it rots and the chicken's breath smells sour.
My chicken Nugget died today from crop problems.
She started with the drool, and would move her head back and forth, then if I picked her up too fast and held her under her crop, she would "vomit". Her crop got really big and swollen, not as hard as a classic "impacted crop" but softer like a "sour crop".
When this happens the chicken usually starts to lose weight, since no food can get past the swelling or obstruction in the crop. They starve to death.
You can check to see if it is a crop problem by feeling her crop in the morning. It should be empty and small (compare how it feels to your other chickens for reference).
Nugget got sick first from eating grass, then straw, then pine shavings, then leaf litter that blew into the run. She just couldn't stay away from shreds of plastic, pieces of old string, or anything else that can clog up a chicken.
I recommend reading about crop problems here on the stickies and on the FAQ and then checking dlhunicorn's site at
http://dlhunicorn.conforums.com/index.cgi.
Good luck, and I hope it is just a passing chicken infection.