Front End Login - Error

[clubbuilder]

I have 2 test accounts on my website (https://www.vetmax.com/) so I can login as customer and test out my cart. My problem is that, when I try and login using the front-end login area, I get a white page with this error

{"error":true,"message":"404 not found!"}

I know my login information is correct. Login will allow me to create a new account, but will not allow me to login with my existing account.

I just don't know what to do here or where to look.