The Sandbox doesn't come with a default MxAdmin user. You could use the CreateUserIfNotExists microflow from the community commons in your afterstartup to make sure there's an admin user present in your app. Using that user, you can add other users to your app.
Signing in with your Mendix account only works if you setup the Appcloud users
After startup microflow
Create a new microflow and set it as your AfterStartup Microflow in your Project Settings https://docs.mendix.com/refguide/project-settings#3-3-after-startup
In that microflow, call the CreateUserIfNotExists microflow which is in the CommunityCommons module (you may need to download the Community Commons from the AppStore, to add it to your project. Setup the microflow call, and deploy your project. You can now login with that user created in your After Startup
As MxAdmin you should be able to login. Did you try setting the password?