Login Issues – security redirect

I have been having issues getting logged into my wp site. It is new but has been working until yesterday, now me or any of my other members cannot login. I have a login link in my header to the login page and also have a login widget in the sidebar,

Below is a link to the page it redirects to when logging in and the message it shows.

***”http://www.treasuresweeper.com/login/?redirect=http%3A%2F%2Fwww.treasuresweeper.com%2Fwp-admin%2F”
“An attempt to bypass security checks was detected! Please go back to the login form, refresh the page, and try again.”*****

It doesn’t matter if you refresh the page, open in a new browser or use one or the other of the 2 login places on site.

HELP please, I have to assume it is a plugin issue but since I cannot get in to figure it out, I have no way of knowing.

Thanks