Persuading ssh -X to work from a RasPiOS system

After a recent round of upgrades to both FreeBSD-current and RasPiOS64
it developed that running 
ssh -X  [freebsd hostname] 
gets me logged in, but issuing
xterm -sb
reports
xterm: Xt error: Can't open display: 
xterm: DISPLAY is not set

After looking at the man page I repeated the experiment
using
ssh -Y 
to open the connection, with the same result.

The X server side has 
X11Forwarding yes
in /etc/ssh/sshd_config, I haven't tampered with anything 
yet on the FreeBSD side.

The display server is on a local subnet connecting via 
router to a public address, but that didn't cause any 
problems before. All addressing is IPV4.

Any hints what might be wrong? I haven't tried this
experiment in a while, so it's unclear which side is
causing the trouble. 

Thanks for reading,

bob prohaska