I have a very old-school indirect xdm/chooser list for a couple of
machines (Linux and FreeBSD). This works fine with the chooser running on
a Linux box. The chooser on a FreeBSD box comes up with the chooser list,
but after selection of a machine I do not get a login window. Instead, I get the message "Cannot connect to xdm" in xdm.log, and after some blinking and flashing
of the screen I get the chooser back. This behaviour is always the same, no
matter if I choose the local (FreeBSD machine) or a different (remote, be it Linux or FreeBSD) one.

I can get a login window from a FreeBSD machine just fine when
contacting it directly  with "X -query <host>", but not going over the
chooser on that machine with "X -indirect <host>". It also works fine using
the chooser from a Linux box, so it looks like something is broken between
the chooser and the xdm login on the FreeBSD machine.
Using Google, I found someone describing quite the same issue (already a couple
of years ago): <https://forums.freebsd.org/threads/5904/>
Unfortunately, there is no solution mentioned there.