Do you assign an Userrole to the new created user?
If not, then that's blocking sign in.
You can assign a userrole as follows:
- Retrieve the required role and set the reference when creating the account
- Retrieve action, XPath constraint. for each role a system variable is create which can be used
https://docs.mendix.com/refguide/xpath-keywords-and-system-variables/#32-user-roles
- Set the association between account and userrole
- Done