apache 2.4.6 virtual host reverse proxy Your browser sent a request that this server could not understand

The case i hit this is due to apache configuration version issue.

Source URL: https://stackoverflow.com/questions/10445455/apache-dont-have-permission-to-access-on-this-server-when-i-am-using-virtua

This configuration was valid for apache version 2.2

First check your apache version by typing apachectl -V in your terminal.

If it shows something like this

Server version: Apache/2.4.27 (Ubuntu)
Server built:   2018-04-18T14:20:05

then your apache version has updated but the conf you are using is deprecated.

So instead of

2.2 configuration:

Order allow,deny
Allow from all

use this:

2.4 configuration:

Require all granted

For further documentation related to the upgrade, follow the official docs:

http://httpd.apache.org/docs/current/upgrading.html