I agree, it's a regurgitation (the smell + appearance are the giveaways...). If he happened to have been slow to digest & staying on the cool side, then yes, it still might have been too close to the previous meal, even at 9 days ago...but it shouldn't have been. Are you sure about the actual temps. in his enclosure, how did you read them? or are you relying on the setting of a t-stat?
You also mentioned he's had some "rancid poops before..." so you really should have his stool checked by a vet, & perhaps the snake too.