Thanks for your reply- there are nine users including myself - I want to add tenth user.
The debug log is:
{2496920:Mon, 16 Sep 2019 17:48:25 GMT} Zenphoto v1.5.4
USER ERROR: MySQLi Error: ( INSERT INTO `[prefix]administrators` (`user`, `valid`, `id`, `pass`, `passupdate`, `passhash`, `rights`, `lastchangeuser`, `date`) VALUES ('sally', '1', '-1', 'q/WNrXoYhGJWtAJOgY5CQaB2sx765W5hLJnC8O/RxCY=', '2019-09-16 18:48:25', '3', '6686561', 'acorkhill', '2019-09-16 18:48:25') ) failed. MySQLi returned the error Unknown column 'lastchangeuser' in 'field list' in /home/acorkhil/public_html/rpsec4/zp-core/functions-db-MySQLi.php on line 93
trigger_error called from query (functions-db-MySQLi.php [93])
from PersistentObject->save (class-persistentobject.php [352])
from Zenphoto_Administrator->save (lib-auth.php [1624])
from admin-users.php [231]
As far as I can tell the website is functioning normally otherwise.