If you find that uploaded images are displaying broken thumbnails, you might find that it is related to your WordPress and Site Address URLs in Settings > General.
If you have an SSL certificate installed, your URL should start ‘https://’ but sometimes your URLs are still set to ‘https://’. (Note the missing ‘s’).
This means that the Add-Ons Ultimate plugin tries to filter out the wrong part of the image URL. To fix, just update your settings so that your URLs are set to ‘https://’.