Hmm – this is strange.
My use case at https://arnowelzel.de/schalterblock-einer-miele-w986-austauschen works exactly the same way. Pictures contain captions as “Caption” in the backend and the plugin has no problem getting the captions from the database and adding them as property to the image links.
Technically:
The plugin now does not parse the HTML code any longer to get the caption but it will try to get the caption from the WordPress database. For this it tries to get the internal post ID on the URL and then uses the post data to get the caption.
This was needed, since the existing gallery plugins for WordPress do not use a well defined way to output the captions, but sometimes add it as aria-attributes, sometimes as figure descriptions and sometimes even as plain text without any link to the picture at all. Also pictures outside galleries may have a caption which is not included in the page at all.
In your case it seems, WordPress can not find the internal post ID for the given image URL. At the moment I don’t know at all what might go wrong. At this point I’d use xdebug to check what’s going in detail on the webserver – but of course this is not possible with your website.
Would you use a debug version of the plugin which collects additional information when passing a certain URL parameter to the page and output that to a log file, which you can send me? If this is OK for you, just contact me via e-mail. You find my address on my website: https://arnowelzel.de/en/about-me.