Hi Armando,
We will look into this.
This happened to me, I resolved by:
1. Setting project level security back to off and running in sandbox
2. Creating MxAdmin user, with pass of my choosing
3, Turning project security back to prototype
It looks like version 7.5.1 < the MxAdmin user isn't automatically created
Problems seems to occur because the generated password is not inline with policy. Turn off require symbol en run again created the demo users