Getting Apache 500 error when using CiviProxy
The key to this would be the actual error message. HTTP Error 500 usually means that PHP encountered a fatal error, which should end up in the webserver's error log. Could you have a look? It could be in /var/log/apache2/error.log, but it depends on your configuration.

Also two remarks (you probably already know this):


The de.systopia.civiproxy extension is only required if you want to send out mailings; it changes the contained URLs to the proxy
Installing the proxy scripts on the same environment is not what we had in mind, the ideal place would be a separate environment in the DMZ.


Cheers
Björn