Request timed out during the communication with PHP process

[dpqmphkncfd71gvxju44]

I need some clarity on this error

[1/11/2013 12:48:55 AM] Error: Request for <PATH>: Unable to get the response within 'Request Timeout'
[1/11/2013 12:49:08 AM] Error: Request timed out during the communication with the PHP process.

This is an ajax request script, it is surely fast, it can't last for 'Request Timeout' of 250 seconds just because php max_execution_time is 240 seconds. According to PHP error log, it did not time out. But according to Apache access log, request lasted for ~250 secs and returned 503. How it could be?
Maybe it's when user disconnected while request was in progress?

[dpqmphkncfd71gvxju44]

What's most interesting is that script's job is done, it goes all the way to the last line and reports about it. Nevertheless, the server "waits" for something and returns 503. It happens only to ajax requests that don't have a response.

[dpqmphkncfd71gvxju44]

Moreover, if I set a very small time limit of 5 sec. within the script itself, it never exceeds it, but "Request timed out" messages keep appearing.