Yes, and no! I was trying to have my Private Messaging scenario display that you had to log in, if you were not already logged in, before the user started the program. The main world constructor would set the new world 'NoGoWorld' to display the message. It worked fine on my laptop (if no user was logged in, the new world displaying that you needed to log in showed). However, when I uploaded it onto the site, I just got a blank screen. If you want, I can upload a simple example for you to look at.

This was said in haste. Still, there were no error messages. But, the program could not have 'stopped' execution because it was not actually started yet. Only compiling and world construcing (possibly) were done.

I am not quite getting the same results, but still they are different. The scenario is here. It is supposed to show a gray screen with the message "Welcome to my world" if you are logged in; and a red screen with the message "Please log in to continue" if you are not. On my laptop, if I compile or reset, the appropriate screen displays; however, on the site, only the gray screen displays, whether logged in or not.

It shows the "Welcome to my world" message when I view the scenario online. So, I'm not seeing the same problem as you - it must be something to do with your browser / Java plugin version. Perhaps try clearing your Java plugin cache.

Ah, sorry, I misunderstood. You are no longer getting the white screen at all? Regarding it showing the wrong result, that was a bug on the Greenfoot side; it should be fixed now, but you may need to clear your Java plugin cache (see link above) before you notice the effect.