@Henk_van_Dijk Your screenshot shows a “files” folder. Is that name deliberate because when you browse for an image the default folder name is “image”.
Can you try using the web console in Firefox, hopefully something similar if you are using a different browser, to see the network requests. When the browse window is open then start network tracking and refresh the window with F5. That might show something about the actual requests and their responses.
I should have referred to PUBLIC_PROTOCOL instead of $public_scheme. Are the public pages, subscribe, preferences, etc. also using https? The plugin uses that when it constructs the url for the image upload directory.