Looks like the app doesn’t support SSO. I finally got it to work by manually running the user creation command from the Cypht documentation:
# create an account
php ./scripts/create_account.php username password
# delete an account
php ./scripts/delete_account.php username
# change an account password
php ./scripts/update_password.php username password
To run these, I logged in to my server as root (i.e. as a normal user and then using su and typing the admin password). From there, you can run
sudo -u cypht bash
to open a shell as the cypht user. (This might be cypht__2, cypht__3, etc. if you have multiple installations, but in the case of Cypht I think that’s highly unlikely 
).
As cypht, I did a cd to switch to the Cypht home directory (/var/www/cypht in my case) and then ran the PHP command to create a new user.
@ericg it might be worth adding this to the documentation! Feel free to copy text from here, or I can write up a proper PR next week when I have time.