Not sure where to place this informatin, so I decided to add it here:
For MODX Revolution 2.7.3-pl
Adding a new backend user with the option “enter password” fails. However, when letting MODX suggest a password, the user will be added to the database.
The steps in detail:
Main Menu: Manage > User > Add new User (green button)
The Domain should end like this:
Add the minimum information at the tab “general information”.
At the bottom of the form, at the right column, check the radio option “enter password”.
Press “save” (green button at top).