Sometimes instead of web pages with data, you only see blank web pages. Some of the reasons for this could be :
1) The PHP code does not execute mid-way through rendering the page server-side so it crashes returning null headers back to the client browser.
2) The server the website is running on has no memory left to process the request , this is an intermittent problem.
3) The website is running on non-server grade hardware that does not have ECC RAM and/or production grade CPUs with adequate capacity. So whenever there is memory corruption or bad bits the Apache process fails midway.
In each case, you can try again after some time.
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