jaswrks
commented
7 years ago @YuqiaoS Thanks for reporting! :-)
Empty responses in PHP applications are generally caused by fatal errors or exceptions that occur at runtime in a part of the code, be it in Comet Cache, or in another plugin running in concert with Comet Cache. Have you reviewed PHP error logs for any clues? Or have you enabled WP_DEBUG mode so that PHP will report errors to you instead of just the empty response?
We've been having issues with empty response errors and they seemed to be caused by Comet Cache. At this point, I think when Comet Cache bugs out with one page or maybe ajax request, the entire backend ajax requests are bugging out. The empty response comes from pages themselves ( not just xhr ) but sometimes adding a '/' at the end of the URL fixes the issue (loads the page). What could cause Comet Cache to give out these empty responses?
We have many sites that have the same issue. Recently we thought it could be WordFence as sometimes deactivating it and activating it solves the problem temporarily but it could just be that deactivating the plugin clears the cache, but after some further testing and research it seems that it's just Comet Cache sometimes bugs out with Ajax requests and the WordPress heartbeat requests all return empty_response_errors.