SSL problem

We have been fiddling with the site to try to make it faster and we succeeded, partly with Autoptimize.
But the calendar page stopped rendering correctly, due to about 30 errors like this:

:1 Mixed Content: The page at ‘https://xxx.org/calendar/’ was loaded over HTTPS, but requested an insecure font ‘http://xx.org/wp-content/plugins/revslider/public/assets/fonts/revicons/revicons.ttf?5510888’. This request has been blocked; the content must be served over HTTPS.

When I turned off CSS in Autoptimize, those errors went away and the page rendered fine again. But we took a hit in speed, of course.

Any solution to this? Why is the CSS asking for unsecure fonts?

THANKS!

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