Why No Padlock?
I play Webmaster for a client who has three sites. Since Google, and others, have been pushing for secure sites for some time now, I recently migrated one of the sites from http to https.
It's a small site and total elapsed tine for the project was about three days. Every page on the site required updating, including all of those 'link rel="canonical"' tags that I had so carefully added a few years ago, along with some scripts and CSS and JavaScript files.
A Web-based tool that I found useful, especially for checking PHP scripts (used for CAPTCHA, forms processing, etc.) was Why No Padlock?
https://www.whynopadlock.com/
Checking the secure Radio Reference login page at https://forums.radioreference.com/ shows three insecure items, as follows.
http://vb-api.com/tracking.php?id=4
http://edge.quantserve.com/quant.js
http://pixel.quantserve.com/pixel/p-68CYS6s2Qv4cM.gif
I tested each of these three URLs using https and they all work just fine, so I'd suggest that all three of them be updated in the Radio Reference code to use https rather than http, as this should eliminate some of the "mixed mode" error messages that people are seeing.
Good luck!
Frank.