Specifically, Chrome throws an error about http response splitting due to a repeated location header when you try to continue to your chosen webmail program (well, Roundcube, havn't looked at the others).
Looking at the LiveHeaders plugin in Firefox (which doesn't suffer the same fate) I don't see a repeated header, but do see two requests which result in Location: header redirects (ie, you get redirected to roundcube's login [automatically logged in from the cPanel details], which redirects again to roundcube's inbox).
I use Chrome 17.x and see the problem, a client uses 16.x and sees the problem, while another fellow uses 14.x and does not see the problem.
Also checked in Chromium 14.x, no problem there either.
... 
