You mean the default Discourse lightbox correct? or are you using a custom theme component like this one?
If you mean the default lightbox, I just tested it in my site (2.7.0.beta9), with the same settings you shared, and it’s working correctly. After you upload an image in the forum you have to wait a few seconds for the lightbox to load, or you could try refreshing the page. Also, depending on the size of your image, there may or may not be a lightbox. You can see an example here in Meta:
With small images that fit completely in the topic there is no lightbox:
But bigger images are available in a lightbox:
Could you check again and if you still have trouble with lightboxes:
- Please share a link to a topic in your forum where the issue is happening (if possible)
- Check your error logs at https://your-site-domain/logs/
- Check if you have any custom theme or custom configuration that may be interfering with lightboxes
- You could also check in your browser console (press f12) to see if there are any errors.