I can tell you what the issue may be. When you have an https site, the browser wants ALL the links on the page to be https, not http. So, people who have signature links that are http, they will cause a problem.
ACTION POINT: I encourage all people with http signature links to change to https if at all possible. Your site may need to be adjusted for https. The reason is that some browsers may suffer this issue as it is then deemed Mixed Content (i.e. secure and unsecure links on the same page). Also, for visitors from searching on Google, they will not find the threads so well that are unsecure, as Google applies a penalty in their algo to pages that are not https.
This situation is likely to become more prominent over time, as Google increasingly applies pressure to make the web secure.