Hmmmm, those kinds of errors can be hard to track down. The first think that comes to mind is that there is a firewall somewhere, that blocks the machine’s external interface from internal communication. I’ve been there, done that, spend hours on it without even getting a tee-shirt
If the request was completely internal, you would have seen it connecting on 127.0.0.1, so it is an internal request, trying to the external interface, then getting blocked.
If anyone else sees the same issue, it’s a permissions issue on the https certificate - the solid user I set up didn’t have access (root only) so the server (silently) failed to start. Fixing the permissions solved the problem.