Transmission reports "Peer listening port" is "Closed"

After switching Transmission to run on a different Ubuntu host on the same LAN, it's now reporting that the "Peer listening port" is closed, however the port is being forwarded on the router and canyouseeme.org reports the port as being open. Transmission is also unable to downloads torrents.

A few more details:

Ubuntu: 23.04
Transmission: 4.0.3 running on Docker
Router: Unifi UDM-SE

The router port forwarding rule seems to be working, as if I disable the rule then canyouseeme.org confirms the port is closed. In any case the rule is a copy of the rule that was working for the previous host (albeit with the Forward IP address changed).

The Ubuntu firewall (ufw) had rules for the Transmission ports, in any case I've disabled the firewall until I can work out what the issue is.

The docker compose yml is the same one I used previously. Can post the yml if it helps.

I'm also running Pihole on Docker (V2023.05.2) on the same host - it's used for ad-blocking on my LAN. I don't think this is a factor as it's been running it for years and the issue persists if I disable the ad-blocking.

Edit: one more thing to add - when I open the Network tab on the Preferences screen in Transmission, the Port status first shows ""Port is Checking...". It takes an unusually long time before it updates to "Port is Closed" - around 18s.

Fixed. The problem turned out to be that I had copied the UUID and PGID (both 1001) in the docker compose yml, and on the new server I needed to use 1000 for both.

No idea why this manifested itself as a Port Closed error, or how I would have diagnosed this, short of double-checking everything.