I now realise my mistake.
I had the password previously set as in the instructions. During all my troubleshooting, at a certain point I threw away everything and started over. So the password was no longer set.
So for Synology users, setting -securitytypes VNC is the correct answer.
Thanks for the help. I use this to backup multiple IMAP accounts to my Synology, combined with filesystem snapshots for version control.