Portainer running but its web UI won't show up

In short: What can one do if Portainer's "Open web" does nothing? OMV6, Docker and Portainer all running (fine otherwise).

More details: Coming from OMV 3 & 4 (more generally Linux with little gui). My main goal is to have a small Syncthing instance running.

OMV 6 installed on a RPi 3B. One external HDD with a single shared folder.

Docker 5:20.10.16~3-0~debian-bullseye installed & running, configured with containers space on the external HDD. Portainer is "up since 26 hours".

# docker ps -a
CONTAINER ID   IMAGE                    COMMAND        CREATED        STATUS        PORTS                                                                                            NAMES
4a8903caba76   portainer/portainer-ce   "/portainer"   27 hours ago   Up 26 hours   0.0.0.0:8000->8000/tcp, :::8000->8000/tcp, 0.0.0.0:9000->9000/tcp, :::9000->9000/tcp, 9443/tcp   portainer

That works, after restarting Docker to restart Portainer as the later required.

ty macom for sharing some of your brain power (am getting out of Covid-19 and certainly doesn't feel o/c