X Input extension not available (?!)

[timotheosh]

1. Running FreeBSD 13.0-RELEASE, with intel 4600 card, KDE Plasma desktop (v5.22) on laptop (heretofore referenced as "the client" or "the laptop"). Everything is working great.
2. Install FreeBSD 13.0-RELEASE onto a Raspberry Pi, and/or 12.2-RELEASE in a jail on TrueNAS, set up sshd and X forwarding, and I am able to open an xterm on a remote host, and view the X application on the laptop. So far so good.
3. Install tigervnc-server and tigervnc-viewer on the remote FreeBSD instances. "pkg install xorg tigervnc-server tigervnc-viewer"

I do not want to connect directly with the vnc client on the client machine. The goal is to have Xvnc listening for connections on localhost only, and the start up vnc viewer locally and use it via X forwarding over ssh to the client machine (my laptop running FreeBSD).

This all works using my Linux laptop as the client. But when I connect using my FreeBSD laptop, I get a GUI window with the message

X input extension not available.

I get this message, even if I open a remote xterm first and then run vncviewer from there. Like I said, this only happens on "the client" and I do not have this issue on a Linux laptop that is similarly configured. So the problem does not appear to be with the remote hosts that are running vncserver and vncviewer. I am assuming there is something else I need to install on the FreeBSD client laptop for this to work as expected?