I checked.
What was kinda weird was that it sent two requests.
In CWP Apache log i found that first response was ok, then CWP resets the connection, and then send the "Username already in database" error because the request is called again.
I think (it's only a thought), that something went wrong in API, and before setting password for the user it ended. Weird is, that it's working in a browser.
What is working correctly is creating a user, everything even with this error, but the user has no password set.
When i was looking in messages log, it says that it could not find a passwd file, i tried to look in CWP code, but it is encrypted/encoded, and i respect the rules, so i did'nt dig deeper, then i was looking for a workaround.
Hope it helps :-)