Login problems with WordPress 4

Just upgraded to WP 4.0 and am now having real problems with logins. Also installed a new theme (Academica) problems started with the Theme preview. When I tried, I had to log in again and the login seemed to be accepted but then returned to the login page. After activating the theme (possibly a mistake?) I can’t login at all. Forced a password reset via email and got back in, and am having the same problem. Have resorted to clearning all cookies and even flipping between Firefox, Chrome and Explorer – currently logged in with explorer but it seems something is wrong. Has anyone any suggestions or seen similar problems? Advice much appreciated.