The most likely reason is that you are embedding your forum in a site delivered by https and you have not yet done what is needed to deliver your forum under https.
I have some guidance at:
http://greghelp.991552.n3.nabble.com/Embedded-Forums-and-https-tp4012846.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.