Fresh jitsi-docker-compose installation with whiteboard: whiteboard only works locally (no collaboration/visibility for others)

I installed jitsi-meet on a vps (with a custom systemd service that runs docker compose and only uses env vars for config), which is working well.

I added the whiteboard by setting the env vars:

WHITEBOARD_ENABLED=1
WHITEBOARD_COLLAB_SERVER_PUBLIC_URL=https://whiteboard.meet.mydomain.com

(where the public URL for the jitsi meet instance is at https://meet.mydomain.com)

When I start a meeting and open the whiteboard from one host, it seems to work well (i.e. I can use the whiteboard), but other participants cannot see it or collaborate. (A tile called “whiteboard” is visible for them, but has no content and cannot be selected).

I’ve also seen it happen that when a second participant enters the meeting whe the first was showing a whiteboard, now only the second user sees it and for the first one it disappears.

I don’t see any suspicious errors in the container logs.

EDIT

I just pulled the latest images (unstable), the former ones were about 3 weeks old. Now all participants can see a whiteboard, but they can only see their own changes, i.e. there is no collaboration.

(Another thing that was a bit troublesome and also suspicious, since nobody mentions this in the docs, is that the built-in letsencrypt functionality doesn’t support multiple hosts (the extra whiteboard vhost), so I had to turn off letsencrypt in the container and use a standalone service for that. But now both the main meet domain and the whiteboard vhost have valid ssl certs)

Am I missing something?

Cheers