After a sucsessfull install process I could'nt reach the https login page because I got a browser (firefoxx) warning that the ssl cert has a wrong serialnumber and I could not log in.
In FireFox, go to the Edit->Preferences->Advanced->View Certificates and remove the previously generated Certificate. And restart Firefox.
You get this problem, cause you already permanently accepted the certificate from the previous install of ISPConfig. And the newly generated cert for the same system is different, causing an alert like this.