HowTo fix: client denied by server configuration?
See Access denied errors by server itself on uploaded files, which you refer to in your question. The answer there applies to the ConfigAndLog file too. Civi periodically runs tests to make sure that certain directories and files are not accessible via the web. These include:

the current debug log file (ConfigAndLog.xxxxxxxxxxxxxxxxxxxxxxxx.log)
the image upload directory (on your Drupal site, this is sites/default/files/civicrm/upload)
the custom php directory (on your Drupal site, this is sites/default/files/civicrm/custom)

The &quot;client denied by server configuration&quot; entries in your error.log are an indication that those directories and files are protected as they should be.
As the author of the security-check function writes in a code comment:

Per CiviCRM default the logfile sits in a folder which is web-accessible, and is protected by a default .htaccess configuration. If server config causes the .htaccess not to function as intended, there may be information disclosure.
The debug log may be jam-packed with sensitive data, we don't want that.

In other words, it's good that the debug log is not accessible via the web.