port 80 **** Hit EOF while fetching headers

Hi, Experts:

We built our system using apache + django and we have lots of clients sending updates to our backend server at around 1min interval. But we do have 9000 around devices. From time to time, we are seeing port 80 **** Hit EOF while fetching headers in apache log or 500 error, could anybody tell me what does this mean and what maybe the possible reason to cause this? or any suggestion what I could do to isolate what cause this issue.

thanks in advance.

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