Zend Server, Dev (Standard) License
After setting up a password for the Developer user in the initial run, the developer user is disabled (cannot use it or modify its password).
This is by design. On a Development Edition, all users are Admins, including the developers, in order to utilize all Zend Server features for matching a production server.
On a production Edition (Small Business, Professional, Enterprise) there is a separation of Developer and Admin capabilities in order to provide more management tasks to the Admin user, while still allowing the Developer user to check Monitoring events, perform code traces and debug the PHP applications on the same or alternative server.
If you need to follow an Admin <-> Developer relational auth schema in a Development / Staging environment, you need to register your server with a Developer - Enterprise license, or upgrade to one of the Production Editions if the server objective is more of a production manner.