Strange behaviour when required fields not completed

On the sign-up page I have about 8 required fields. Take for example the First_Name and Company fields.

If I don’t complete the Company field I get “The Company field is required,” and see the form again with required field highlighted. But if I don’t complete the First_Name field I get sent to a 404 Page Not Found error page!

If I complete neither I get “The First Name field is required. The Company field is required.”