okay, that is really puzzling.. Okay, let's try something ..
I've set it up so that depending upon the preset selected, it will disable certain parts.. so hopefully this will help find the location where it is breaking..
I need you to make a note what happens when you do each of the following steps..
If you go to the page again and click on the preset 1 button( the number 1 button below the color sliders), and then try drawing, it shouldn't draw because everything is enabled.
Then click number 2, and try drawing.. let me know if it does or not.
Then click number 3, and try drawing.. let me know if it does or not..
then click number 4, try drawing, etc..
then click number 5, try drawing, etc..
then click number 6, try drawing, etc..
you only have to go up to 6, but let me know when you're able to draw (also make sure your tool is set to pencil while you do this).
If for some reason you can't draw on any of them, try the process over again, but resize the canvas by dragging the resize handle after you click a new preset button. So, click 1, resize, draw, click 2, resize, draw, etc..