Using Drush to update to 8.1.8: 500 Internal Server Error

I installed a Drupal 8 site in June (sfwalk.net) as a way of learning the platform, and just tried to follow this procedure to update to 8.1.8.

I follow the procedure exactly, including copying over my existing settings.php file and setting permissions.

I get the following messages from Drush:

Project drupal was updated successfully. Installed version is now 8.1.8.
Backups were saved into the directory /home/yeaton/drush-backups/yeaton_sfwalk/20160808172259/drupal.   

Now the site throws this error:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@sfftp.yeaton.net and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

I checked the server log file using cpanel, but it's empty.

Not a big deal to write over the site and start over (no content is stored locally) but I'd like to figure out where I went astray. Thanks for any suggestions.