Images show as unloaded icons

Hi,

I have set up the plugin, with my slider and all my images set up via the plugin’s settings, as well as the image sizes and the links to the images. However, the slider shows my thumbnails as unloaded image icons in the plugin’s Slider Preview and in the article I put the shortcode in. When I click on the icons, the images do open. So why are the images not being displayed in the slider?

See my slider here:
https://support.ubity.com/?ht_kb=c-0002-brancher-un-routeur-asus-derriere-une-homehub&lang=fr

https://www.ads-software.com/plugins/wp-responsive-thumbnail-slider/