I can't login with any other users either. I think it's because I've changed the password in ISPconfigs config.inc.php file, both server and client.. So I guess ISPconfig (and my mail etc) can't connect to mysql to check if the username / password is correct)
Yes,you locked yourself out of the mysql database now. Never change the passwords in that files, they are managed by the ispconfig installer and the same password is used for many other servoces like postfix tc as well, so dont change it. You can find the corrrect password in the mysql*...cf config files in /etc/postfix/ directory. Take the password from one of these files and set it again inb the ispconfig config.inc.php file.