apt-get remove python-six

Unfortunately 10.04 isn't supported (as said on the download page), but you still can install Ajenti through PIP and it will work:

sudo apt-get install python-pip libxml2-dev libxslt-dev libmysqlclient-dev (please check exact pkg names on 10.04)
sudo pip install --upgrade ajenti

Have you had any luck with the certificate so far? Arkos is in fact an Ajenti-based software, and the problem reported in the above link looks identical to yours.

Added PM selection to FPM options

Doesn't the forwarding work for you? You should be able to add a forwarding entry pointing to a local address.

Could you please elaborate?

It totally makes sense to do the root redirects, as / is the entry point of this web-application. I've updated the page with a working redirect-rewriting config:  http://support.ajenti.org/topic/472359-ajenti-behind-apache/

Thank you for the input! I've simplified your config a bit, fixed the ProxyPassReverse directive, and now everything works, including the login redirects: http://support.ajenti.org/topic/472359-ajenti-behind-apache/