Hello,
no, so first of all, don’t blindly configure user and group everywhere without understanding, you don’t need that at all actually. The other examples you found are about haproxy listening on unix sockets, which is not your case.
You never need user/group settings on IP socket or abns sockets, because they cannot even be applied, user and permission settings are about files (a unix socket is a file, and abns or IP socket is not).
The is no backend user/group setting either, this doesn’t make sense.
You DO need to check permission of the unix socket and understand it. But ustreamer.sock is world readable, so you are already ok.
I hope you didn’t just move the file. You had the application reconfigured with the new path, right? You cannot just move a unix socket around, if the application is listening on the old path.
You can hard link the socket from the chroot location, or tell the application to listen in there.
That’s the wrong path. Assuming your chroot is /var/lib/haproxy/ (but you omitted the chroot configuration so I can only guess), then your path is /ustreamer.sock:
server rpicam /ustreamer.sock