The simpler media website CMS
I see you have had queries in the past about this error - without being able to provide a cause or solution.
I have now come across it for the first time - trying to add a second user produces this error.
I have another zenphoto database and app installed in the same web site - it has no problem with adding more users, so the message is not one appropriate to the error, whatever that might be.
Your help would be appreciated.
Zenphoto version 1.5.8b (Official build)
Current locale setting: en_US.UTF8
Current gallery theme: zpBase
Server software: Apache
PHP version: 7.4.13
MySQLi version: 5.7.32