Content Encoding Error - The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression Drupal 7.56, PHP 5, PHP-FPM, multi-site Ubuntu 14.04.1 LTS server

EDIT: it's NOT only when I'm logged in to my site do I get this error:


  Content Encoding Error
  
  The page you are trying to view cannot be shown because it uses an
  invalid or unsupported form of compression.


It happens 1/3 of the time I try to load a page. If I click "try again" twice or refresh the page twice it will load. I have followed these steps from the accepted answer on Content encoding error after enabling cache, aggregation and compression

$conf = array(
'cache' =&gt; '0',
'preprocess_css' =&gt; '0',
'preprocess_js' =&gt; '0',
'block_cache' =&gt; '0',
'page_compression' =&gt; '0',
);


I have also checked php.ini, per other resolutions, and error reporting is off.

I checked for PHP error reporting per https://www.drupaleasy.com/quicktips/browser-fail-unsupported-form-compression-related-php-notice and https://www.drupal.org/node/373101 and when I ran the wget command there was no PHP error up top.

The behavior does not change at all after clearing my browser's cache.

FWIW I've tested by disabling every module I could (I'm fighting AddThis right now). I also had to increase the memory limit from 128M to 1000M because I kept getting 500 errors too. See my note on 500 Internal Server Error after logging in 

I also had to update a theme.inc file based on this error: 


  /e modifier is no longer supported, use preg_replace_callback instead
  in…


as seen at https://stackoverflow.com/questions/38067166/replace-preg-replace-with-preg-replace-callback-still-failing?noredirect=1&amp;lq=1

UPDATE:
the 500 error seems to have been fixed with a permissions reset. I put the memory_limit back to 128M after the permissions reset and no more 500 errors. I got one from Chrome at first but maybe it was a cached thing? Now no more.

Also, per Anson W Han in the comments, I have tried reverting my change to the theme file with no success. And the theme is supposed to be best with PHP 5.2-5.6 and I am running PHP 5.5.9 

UPDATE:
the 500 error is NOT resolved after all
Looking in the log file there were some FastCGI references, but in ISPConfig I had PHP-FPM selected. Once it was switched to MOD-PHP all of my problems disappeared.
I can't even.