Have you added https redirects into .htaccess yet?
You'll probably want these lines:
Code:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.yoursite.com$ [NC]
RewriteRule ^(.*)$ https://www.yoursite.com/$1 [R=301,L]
RewriteEngine on
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
Replace
yoursite with your website URL. Switch your forum URLs back to
https://www.
Try logging in now
--------------- Added [DATE]1381800610[/DATE] at [TIME]1381800610[/TIME] ---------------
Edit, ozzy's answer is the right one. Oops!!