Login Issues

Hi there, I seem to be having a small issue with the plugin. When logging onto the site I’m working on, it works on multiple computers, but just one computer is sometimes unable to login successfully. It redirects back to the login page but that’s all. We cleared cache, and it worked fine for two attempts, but afterwards issue arises again where we cannot login. Any help would be extremely appreciated.

Thanks.

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