Unable to Update to WP 4.9.6

I’ve tried to update 6 of my sites to 4.9.6 using the Update button on admin. All I get is a 500 Internal Server Error after about 2 minutes. Problem has persisted for over 12 hours. Doing this does NOT result in a locked website.

I tried disabling all plugins and renaming htaccess but still no success.

I tried turning on WP_DEBUG (true). I then ran the Update to WP. Instead of running for 2 minutes it just returned after a few seconds but did not update anything and no obvious error messages were displayed.

I tried updating a theme (twenty fifteen) and that worked successfully.

My sites are all running on the following:
PHP Version: 5.6.27
MySQL Version: 5.5.43
Webserver Info: Apache

I’ve seen at least one more post from someone who reported that none of their websites were updating “automatically” so I have not tried scheduling the auto update.