Updated on Feb 8, 2017:
added tips to install Seafile Pro and to set up a Startup Cron Job
DISCLAIMER: I am not an engineer nor a professional sysadmin. Use this guide at your own risk. You may loose data, fingers and other parts of your body, and your brain may be eaten by zombies. If something bad happens, do not complain with me.
Seafile is a great piece of free software: it allows you to have a dropbox-like service hosted on your own server, which means that your data are under your control (we know that Dropbox has already been hacked once in 2012, and we cannot be sure that it has not been hacked again since then…)
ISPConfig is a great piece of free software, too: even if it is designed as a tool for ISPs who sell web hosting services, it is useful also if you want to run your own server and you want an easy and powerful tool to manage websites, email accounts, mailing lists, etc. as well as to manage the server itself.
There is an old guide on howtoforge.com to install and configure Seafile on ISPConfig, but it has not been updated and it does not work with most recent versions of Seafile.
So, here is my guide to install Seafile Server v6 on a server running Debian 8 (Jessie) and ISPConfig 126.96.36.199p9 with Apache (it should work also on ISPConfig 3.1, but I did not tested it yet).