I solved it! The error above was overcome by fixing the permissions on /tmp (it was not writable for some reason). I found that this was an issue by running Xvnc manually and seeing that Xvnc failed to create a “/tmp/.tX1-lock”. There was no other system error log, so it was tough to figure out until I did this manual run.
After that, the “Launch Desktop” button was finally visible. But, clicking it resulted in “Failed to connect to server” in the VNC console. By reading a prior topic here, I found that there was a 404 code in the Websockify request. After configuring the reverse proxy, the desktop finally works!
Thank you for your help, Jeff!