Switched from http to https, complains about loading mixed insecure content on a secure page. SSL?

Question

Digital certificates

I didn’t find such info, I found a similar one that swears at an advertising banner with http, but I figured out I fixed the problem with editing style.css, I tried it but it didn’t help, the toist didn’t find the background.jpg there, it’s different for us. I was looking for that file in the file manager, I found it seems to have deleted the same thing. I’ll say right away in Wordpress in the settings where they say for dummies it’s already https instead of http.
I tried through the current plugin, aggravated it, eliminated the site completely with various errors.
1) In general, the browser issues this
Loading mixed (insecure) displayed content " site.ru/wp-content/uploads/2018/08/Background-Digi... " on a secure page
or
Mixed Content: The page at ' https:// site.ru/' was loaded over HTTPS, but requested an insecure image ' site.ru/wp-content/uploads/2018/08/Background-Digi... '. This content should also be served over HTTPS.

and 2) I am using a free issued certificate from Let's Encrypt hosting, everything is in order in the hosting control panel, or rather, it gives out such information. But only Chrome swears like this
This page is not secure (broken HTTPS).
Certificate - missing
This site is missing a valid, trusted certificate (net::ERR_CERT_DATE_INVALID).
If you look at the certificate, it has already expired 3 months ago.
This I think I'll figure it out, I'll talk to the hosting, I'll renew the certificate, or I thought it will be automatically updated by Beget hosting, but is it possible even if it has expired, show it as if it's working properly?

Answer 1

Obviously, somewhere there is a connection of resources via http - maybe pictures, styles or js - if you yourself don’t see it in chrome-dev-tools, then show a normal unbroken link, and they will show you a finger.
ps. you can still like this - it's quite simple: in chrome on the page, anywhere on the problem page - right click - view the page code. On the ctrl+F page that opens in a new tab at view-source:https..... page - search on the http://
page. All that you found - we are already looking in the vp, where to fix it.
I assume that in articles or in posts there are links to http.