Maybe it is not mixed connetent but missing ocsp response ?
If you have firefox can you check with F12 or CRTL+ALT+J if there is any security warning ?
bout:config
- security.mixed_content.block_display_content
- security.mixed_content.block_active_content
- security.warn_viewing_mixed.show_once
- security.warn_viewing_mixed
Could be also interesting.