Originally Posted by till
You can reproduce which behaviour?
I use firefox and the developer toolbar since it is available and never had any problem with the ispconfig login.
The same for me. Except I enable the "show validation bar" of the web developer plugin (in german it is: Extras => "Seitenvalidierungsleiste anzeigen"). Instantly the admin login is not possible any more and I get "login.php?err=" in the address bar. Please try it out.