The ProteinVR app at durrantlab.com uses the HTTPS protocol. Some users may wish to load PDB or SDF files that are hosted on websites that use the less secure HTTP protocol. Unfortunately, modern browsers don’t allow HTTPS websites to load HTTP content. If you are the webmaster running an HTTP site, I recommend switching your website to HTTPS. Now days you can do it for free using Let’s Encrypt: https://letsencrypt.org/ .
Future versions of ProteinVR will provide a more specific warning when trying to load HTTP content.