Getting mixed-content errors even though I’m only using https URLs

I'm having the SSL warning messages all over my website after switching to SSL for several assets:

Mixed Content: The page at 'https://example.com' was loaded over HTTPS, but requested an insecure script 'http://example.com/script.js'. This request has been blocked; the content must be served over HTTPS.

I checked the page source, every single script/css is requested over https.
I even checked the dynamically created html by using the code inspector.
I disabled Javascript in case a script was loading these assets dynamically.

None of these things showed a single http:// request. I'm out of ideas to try and find what is causing this. Any ideas or suggestions?

Solution

When seeing a mixed-content message about a http://example.com/script.js (non-https) URL that doesn’t actually appear anywhere in your sources, the basic strategy to follow is:

Replace the http in the URL with https and put that into the address bar in your browser: https://example.com/script.js
If your browser redirects from that https://example.com/script.js URL back to (non-https) http://example.com/script.js, then you’ve found the cause: example.com/script.js isn’t actually available from an https URL, and ends up getting served from a http URL even though your source is requesting the https URL.