But the suggested address for the thumbnail is correct - that's where the thumbnail image is. Copy it, and paste it into your browser!
Yes, that is the thumbnail suggested by the client when the album was uploaded. Since that one failed at upload time the server doesn't recheck it now. It won't happen now since webp will work when uploading.
Just tried to change the URL now (from thumbs to slides) and click save and that made the thumbnail work.