When you create a user with a mixed case username it is stored as all lower case. When you try and login with the mixed case username it will fail, but logging in as the lower case name works. This is a change from the 2.1.x versions that are released where mixed case usernames worked.
CREATE USER stBarts WITH PASSWORD 'island';
The above statement changes the username to "stbarts".
This would not be so bad except during login case does matter and has to match what is stored in the system.
Recommended fix: allow mixed case usernames to be stored in system, or convert mixed case username entered to lower case during login.