Ok. Then there might be a problem with the session. Close all browser windows and then start the internet browser again and connect to ispconfig.
If this does not help, download the latest ispconfig again (no problem if its the same version that you have installed) and run the update.php script. Choose 'n' when you were asked if your services shall be reconfigured so that only the ispconfig files will be copied again.
Not sure what causes this on your setup. Making a new theme by copying the default theme should work, but I will try it on my test server.