AX2012: The default login company does not exist

When you are facing the error message "The defaul login company does not exist" / "Das Unternehmen für die Standardanmeldung ist nicht vorhanden" while trying to open an AX client, you should have a look at the ax client configuration or your .axc file.
Just enter a startup company even thogh the field is empty. "DAT" should alway work.
When your able to login again change the default company for the user.

In my case this error occured due to the fact that my user was deleted from the initial partition and tried to login to company XYZ in another partition.