I've noticed this message in some ajax requests. To reproduce the error, just compile and run the attached example and hit some clicks in the link "Go to url (internal or external)...".
I've temporally removed the flag MHD_USE_DEBUG to solve the problem, but I would like to understand why MHD sometimes reports this error, because I always let the MHD_USE_DEBUG enabled to report other problems.
P.S.: I never use callings like "sleep(1);" in my requests, it was just to reproduce the problem, because I couldn't found an better way to reproduce it. :-/