My Multisite is broken at the sign up process

I installed a plug-in called Block Spam by Math Reloaded the other day and it seemed to work fine. However after a few days I noticed that the signup form was broken. I thought it was the theme that was broken so I changed to another theme, but still it was broken. I finally changed to Twenty Twelve, but that was not fixed. So, I concluded that it was not theme but the site itself that is broken. Here is the broken form . I need your help to fix that, please.