Super RSS Reader displays mixed content for the RSS feed links and images, rather than recognizing the WP site is secure and reformatting the links in the RSS feed.
The mixed content is due to the “Show thumbnail if present” not recognizing the site is secure and updating the http:// URLs to use https:// URLs in the thumbnails.
The only solution to get the site loading securely is to disable the Show thumbnail if present option.
Here is one example:
Mixed Content – Errors
Soft Failure An image with an insecure url of “http://res.cloudinary.com/simpleview/image/upload/v1604325063/clients/lansing/CVB_Farmers_l09222016_135_9d300133-818f-4d0b-a1a8-c915d964e125.jpg” was loaded on line: 174 of https://example.com/.
This URL will need to be updated to use a secure URL for your padlock to return.
The image above can be loaded securely if the URL is changed to https://
Would it be possible to update Super RSS Feed to see if the site is loading securely and update the URLS for thumbnails to replace HTTP:// with https://?