Greenfoot.start() in the start of the world code makes lines like "if (Greenfoot.isMouseClicked(this))" stop working the first time you reset the scencario, though if you click the reset button again the scenario will work fine. So clicking the reset button only makes the scenario stop working the first time afterwards you can reset as much as you like without any freezes. I don't know why it works like that, but I got the exact same issue with Wombat Puzzles, which uses Greenfoot.start() in the very start of a menu world as well. But in Wombat Puzzles I tried to add a wombat from the normal greenfoot scenario wombats, and when I clicked reset the first time I couldn't click on any button, but the wombat was running as it should.