Did you try logging in with your Mendix account? In the Sandbox environment no MxAdmin account is created.
Hi Stephan,
There is another way to log in as Admin to your Free App.
This workaround worked for my Free App:
Hi Stephan,
For the first time, password will not be get set for admin , so, give administration module access to another user and go to local users and change the password for admin. So, from then it starts working.
Hi, you've probably more than 10 'named' accounts in your free app. After the upgrade it is not allowed to have more than 10 users. You can build an after start-up MF to delete the accounts (except your mxadmin account).