Unfortunately you do not only have a problem when the password contains a %. After reading your reply to consuella, I went back and re-registered with an email address which did not contain a +, which is entirely legal as an email address and supported and used by me in my Gmail address. After using an email address without a + and creating a new account the Sign in from the program worked.
Please fix your email constraints as well as the password bug.
One can only hope that your adherence to HTML5 protocols and related is more complete.