Tips & tricks:

(1) When using Mozilla Firefox check your browser settings under "Extras > Settings" regarding

• Main (Settings Java und Javascript)
• Security
• Privacy

Ascertain that the checkboxes Javascript, Java and "Load images automatically" have been activated. 

(2) It is possible that cookies cause a concurrent logout during the login-process. In that case clear your cache and cookies.

• Mozilla Firefox: Extras > Settings -> Private data button "Settings", click the checkboxes "Cache" and "Cookies", click "OK" and  "Clear now", subsequently. Confirm with okay "OK".
• Internet Explorer: Extra > Internet settings > Main > clear cookies.

(3) Problematic could also be several "Plug-ins" (which possibly installed themselves). It might be that there is a plug-in that causes after a very short time the site to refresh again, which finally ask for a relogin. In that case, please check your list of plug-ins.

(4) Another, possible, but not imperative origin of the problem might be the firewall, that you are probably using. A delicately one could be e.g.  "Zonealarm", when it was set strongly.