Could the millet and or cracked corn be moldy or contain fungus? And although I’m new at the chicken thing, I tend to agree that it would be strange for them all to have sour crop at the same time.
I have several hens that have HORRIBLE smelling mouths. I mean like a dead animal!
They are all eating, drinking and free ranging in the yard. Their activity is normal, no eye or nasal discharge, no respiratory problems or sneezing.
I have my Silkie in my lap right now and I can smell her...