It's your app, how are we supposed to know? We're just developers like you.
The App Review Team will have taken the credentials you gave them, and tried to login. When the login failed, they stopped reviewing it and informed you of the issue. That'll be the extent of their information on how to reproduce the issue.
Do you have any logging on your server so you know why the login failed? Have you tried to debug it, and follow through the code to see what happens when you attempt to login with those credentials? Whilst stepping through the code you might notice something that could be causing the issue.
Why is the password field blank? Does it get wiped the moment the user presses the Login button? Did the App Review Team attempt to login without a password? Did you give them a password?
So many questions...
Topic:
App Store Distribution & Marketing
SubTopic:
App Review