Whenever trying to create a new user, you get a message saying "The username is already being used".
The usual cause for this is that a authentication app is enabled; e.g. the "WebDAV user backend".
- If you can live without the interfering authentication app, just disable it.
Note: For letting your ownCloud access WebDAV shares, or for making your ownCloud accessible via WebDAV, you don't need this app at all - it is only required if you want to reuse the login facilities of an existing WebDAV server!
- If you absolutely must mix internal accounts with ones from authentication apps, disable those authentication apps while creating internal users, and enable them again afterwards.
For some authentication apps (e.g. WebDAV), there is no way to tell whether a user exists or not (the only thing you can check with it is whether a certain username/password combination is valid).
When creating a new user, the ownCloud user administration interface however needs to know whether a user already exists or not in any of the backends (so as not to "overwrite"/hide any existing user account).
If now, you have some authentication app enabled, the user backend has to tell ownCloud for every username that it already exists - because it can't determine whether it really exists.
The consequence is that when such an authentication app is enabled, you cannot create any more "internal" users.
Generally it is probably good practice to keep a user database in one place only. Otherwise you can easily end up with conflicting entries (i.e. same username but different password).