The setting for Encrypt Login Form in the SCLogin module should work just fine. That setting only pertains to the Joomla login options and has nothing to do with the username/password login only. Social logins will use whatever scheme the current page is loaded over and SSL should work just fine.
Beyond that, can you let me know:
* Where you're seeing the message? Is this in the Javascript console of the browser, or somewhere else?
* When you say "When logging in in the frontend,", do you see this message just during the login process, before you try to login, or after you've successfully logged in?
One way to investigate the issue is simply look at the HTML on the page that is showing the mixed-content message. Then, search for "http://". If you find any images or javascript files loaded over http:// and not https://, that's what's causing the error. Simple links to other pages or even forms *can* have http:// links without causing that warning. It's just the content that's loaded on the current page that will cause the problem.
I hope that helps,
Alex
SSL and JFBC
