callback url not being processed

I have been seeing a similar problem: The callback url IS being accessed, but somehow between last night and this morning, the callback has stopped posting the required JSON. No change had been made to my url code, until after the problem was detected, and even then it was simply changed to dump the posted contents, for debugging: nothing came up.

This is a somewhat urgent problem, because now none of our archives are being processed appropriately. What can we do?

Thank you...