500 Internal server error

Can you make the error happen but this time have dblog/watchdog enabled so that a line number for the error gets outputted?

One thing I like to do is put this in the settings.php file

if (!empty($_GET['display_error'])) {
  // Show Errors in output
  ini_set('display_errors', '1');
  // Report all php errors.
  error_reporting(-1);
  // Display errors using drupal_set_message().
  $conf['error_level'] = 2;
}

Then if you get the WSOD reload the page with ?display_error=1 to see the error code.

I got 500 Internal Server Errors too after updating to 2.20. In my case I found in the apache error log the following error:

sites/default/files/advagg_js/.htaccess: Option FollowSymLinks not allowed here, referer: http://www.example.com/admin/config/development/performance/advagg

New dev release + Setting Use "Options +SymLinksIfOwnerMatch" under Obscure Options fixed it, see: #2850290: Support the "Options +SymLinksifOwnerMatch" option for htaccess files.

The debug-code in #2 doesn't help as there is no error output on 500 error, but it's written to apache error log.

Going to mark this as fixes due to #3