Seems Sling TV has made a recent change to their login page or a domain has recently been added to the blacklist. I tracked down the blocked domain that is causing the log in problem as tags.tiqcdn.com. There is an element on the Sling TV login web page that calls this domain. If the domain is blocked, the element can’t load and renders the login button as useless.
If you whitelist this domain, the functionality of the login button returns and you can login to the Sling TV webpage.