What makes a username or password invalid?

Here are the rules for your hosting user name and password.

Username must be alphanumeric and start with a lowercase letter.
Min length: 3. Max length: 8
IMPORTANT: Username may not contain hyphens or spaces!

Password must be between 3 and 15 alphanumeric characters starting with a lowercase letter.
Note: The underscore and dash characters are acceptable.
IMPORTANT: The password cannot contain any part of the username!