Everything should be up and running and shiny new now!
I've upgraded the server OS from Debian/Squeeze to Debian/Jessie and Redmine from 1.2.1 to 3.2.0.
Besides this, the site now uses a Letsencrypt SSL certificate and any non-SSL requests get redirected to https://.
Please let me know, if you encounter any problems.
Most likely, I need to do some minor tweaks during the next days, so you might encounter short outages.
BR,
Tobias