Unknown error on logon from mobile

[yktdan]

I have installed on my site and enabled and cleared caches. The message "Unknown error" appears when I try to logon. It does spin its wheels for a bit so it is talking to site but then fails. Get the same message if I deliberately use a bad username or password, so it is something else.

access denied
http://dev.atvista2020.org/index.php?q=civicrm/civimobile/auth
I don't see any new permissions that need to be set, but logging on as u=1 admin so that should not be an issue.
On Civi 5.3.1 (not of 5.4 yet)