Q:

Apache Proxy: No protocol handler was valid

I am trying to proxy a subdirectory to another server.  My httpd.conf:
RewriteEngine On
ProxyPreserveHost On
RewriteRule .*subdir/ https://anotherserver/subdir/ [P]

The problem is that Apache is always logging this:
AH01144: No protocol handler was valid for the URL /subdir/. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.,  referer: http://localhost/

So after searching the internet, I have activated these modules:
LoadModule headers_module modules/mod_headers.so
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_ajp_module modules/mod_proxy_ajp.so
LoadModule proxy_connect_module modules/mod_proxy_connect.so
LoadModule proxy_express_module modules/mod_proxy_express.so
LoadModule proxy_fcgi_module modules/mod_proxy_fcgi.so
LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
LoadModule proxy_html_module modules/mod_proxy_html.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_scgi_module modules/mod_proxy_scgi.so
LoadModule proxy_wstunnel_module modules/mod_proxy_wstunnel.so
LoadModule rewrite_module modules/mod_rewrite.so

(I know that I don't need all of them but I just activated them to be sure I am not missing one)
But this error still appears and clients get a HTTP 500. 
How can I fix this? 

A:

This can happen if you don't have mod_proxy_http enabled 
sudo a2enmod proxy_http
For me to get my https based load balancer working, i had to enable the following:
sudo a2enmod ssl
sudo a2enmod proxy
sudo a2enmod proxy_balancer
sudo a2enmod proxy_http

A:

For my Apache2.4 + php5-fpm installation to start working, I needed to activate the following Apache modules:
sudo a2enmod proxy
sudo a2enmod proxy_fcgi

No need for proxy_http, and this is what sends all .php files straight to php5-fpm:
<FilesMatch \.php$>
    SetHandler "proxy:unix:/var/run/php5-fpm.sock|fcgi://localhost"
</FilesMatch>

A:

In my case, I needed proxy_ajp module. 
a2enmod proxy proxy_http proxy_ajp 

