login not available or un acceptable

Hello, when activating brute force with a specific term, when logging in I often get the message: not available or unacceptable. I then need to deactivate the plugin via FTP, log in, activate the plugin and deselect the brute force. My host says they do not cache, so is it a conflict with another plugin or to do with another setting?

The page I need help with: [log in to see the link]