On the one hand you are right. The modeler could have given a better hint on what you are doing wrong. On the other hand if you make a mistake like that you have not read anything on how Mendix handles security. I would seriously suggest to follow the following learining path: https://academy.mendix.com/link/paths/9/Configure-Advanced-Security
Because otherwise you will learn that your anonymous users can do all kinds of stuff you probably do not want them to do. Anybody with a bit of Javascript is able to change your data in the database.
Regards,
Ronald
Found my own answer ..
It is not allowed to use the 'Administartor' role. After selecting another role, that also was setup the same way as the Administrator role, the problem was solved.
While it is understandable you should not use the 'Administrator' rol for anonymous access .. I still would have expected system control or correct log messages that would point this out.