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Strange errors on requests for resources, http instead of https and some net::ERR_FAILED

I manage a site and if I open the browser console with disabled cache, there are sometimes errors in queries on some resources. Among other things, errors are
GET https://www.domainname.topdomain/wp-content/uploads/2018/04/AN-IMAGE-150x150.jpg net::ERR_FAILED
These errors are not always, and not on the same resources. One time there was an http request on the emoji script instead of https. And I then saw in the html code that the address had the protocol http instead of https for this script. How can that be?