Drupal image style issues
It was my sever setup after all. Thanks to this post: Is there a common nginx conf for Drupal 7 sites?, I was able to fix the issue. Image styles now work on all my sites.

My initial setup was 

    location ~ ^/sites/.*/files/styles/ {
            try_files $uri @rewrite;
    }


The above setup worked for a while and was working for one of my sites but won't work after upgrade for the others for some reason.

I changed it to this

    # Image styles for D7
    location ~* /sites/.*/files/styles/ {
            access_log off;
            expires 30d;
            try_files $uri @rewrite;
    }


and it's working now.