I re-generated the server cert as described by falko in this thread http://www.howtoforge.com/forums/sho...58&postcount=4
and after restarting ISPConfig the problem was solved.
(I can now access the admin interface and I got ISPConfig listening on port 81)
tcp 0 0 *:81 *:* LISTEN 22042/ispconfig_htt
Thank you all for your help, I don't regret changing from "the Pleskvirus" to ISPConfig, my best decission ever!