PHP serving blank pages to <1% of users

Solution 1:

Maybe this will help you: Have you KeepAlive to set on?

We had the same problem in varnish with mpm-itk, the problem was, that when mpm-itk was accessed with a different vhost then the actuall keep-alive session was, he just killed the connection. Every normal Browser would then try to reconnect, but varnish doesn't in a default configuration. With removing KeepAlive this behaviour could not happen, so it fixed this problem.

I know, that you are not using mpm-itk, but maybe it is worth a try.

Another Idea: Can you access the site with bypassing the varnish and try to force the blank page? So you would maybe find out if the varnish is the problem.

Solution 2:

This turned out to be an issue where PHP was unable to log to the specified log file due to a permissions problem, and display of errors was disabled.

As such, the PHP error was not displayed, or able to be logged - but the root cause of the blank pages was due to a strange PHP fatal error.