Hello,
would a business rule "After creating a user" be triggered in this way or not
Business rules will not trigger in this case. They only get triggered upon operations performed via Adaxes.
So I created a scheduled task, but there is no option to run certain tasks if the previous one was successfull ... any reason for this?
Scheduled tasks are separate configuration objects not related to other ones or specific operations. As such, there is no possibility to link execution of the tasks based on success. As an option, you can try using a single scheduled task adjusting the blocks of actions/conditions accordingly.