I managed to narrow the problem to this:

If I use locally stored image (eg.: https://my_External_url.com/pictrs/image/ddfdfdfdddd.jpg) as image URL during post creation - Lemmy doesn’t generate thumbnail either.

So somehow lemmy can’t access locally stored image for thumbnail generation.

Workaround could be to configure pictrs in lemmy.hjson as not a local container but as a http://my_External_url.com/pictrs. So it would treat them as external image. Couldn’t manage to test that theory yet :)

I have the same problem. Fresh install of my Lemmy. If I upload image - no thumbnail is displayed. I analyzed the logs and DB. Looks like Lemmy leaves thumbnail_url value “NULL” in the post table. And when external image is used - thumbnail is being generated and thumbnail_url value changes from NULL to an image name.

Did you find a solution?