Troubleshooting the "client denied by server configuration" error.

Symptoms

You are trying to access a part of your website and receive an error message regarding not being authorized, or that you do not have access.  When checking the error relevant error log,(/etc/apache2/logs/error_log usually works) you find errors that resemble the following.

[Sat Aug 01 16:19:19.233863 2020] [authz_core:error] [pid 2129:tid 47453278574336] [client123.123.123.123:47252] AH01630: client denied by server configuration: /home/user/public_html/filename

Description 

This error is generally due to having some kind of "allow/deny" list in place that blocks connections to your domain that come from IP addresses not explicitly allowed in said "allow" list.  This could be in one of your local .htaccess files, or more rarely, in an Apache include.  You will want to look out for rules that resemble this.

<Directory /var/www/foo>
  Order allow,deny
  Deny from all
</Directory>

Workaround

You will want to make sure your IP address is properly added to any relative "allow" list, or remove these rules from your .htaccess file or Apache include.