Well, the error apparently comes from the remote (Jellyfin in this case) doing something with the connection that Caddy (or its underlying Go standard library) isn’t expecting. Caddy itself doesn’t have code that emits that error, so I think it originates from the Go standard library, but I’m not sure what is causing it to happen in the first place. It’s basically something from Go itself or from Jellyfin, or the interaction between the two.
Can you make it minimally reproducible? I have a Jellyfin setup so I would be willing to try.