wp-comments-post.php blank page

After updating from the Responsive Classifieds Theme ver 6.6.3 to ver 8, I notice that posting a comment on a listing returns a blank page, the comment does not appear and you are left on the wp-comments-post.php page which is completely blank.

I have tried disabling all plugins, I only have two which are WP Slimstat and Limit Login Attempts but that had no effect.

History of events:
Upgraded from WP 4.0.1 and CT 6.6.3 to WP 4.2.1 and CT 8. (comments don’t work)
Restored site back to WP 4.0.1 and CT 6.6.3 (comments work)
Upgraded to WP 4.2.1 (comments still work)
upgraded to CT 8 (comments don’t work).

Can anyone help?