CREATE DATABASE permission denied in database 'master'

Hi, my situation is like this. Initally, all my web application and setup works. But due to some issue in my SQL Server which I can't solve, I created another instance database. But when I updated back my web.config, my application work but my authentication built in code doesn't work. The error is
CREATE DATABASE permission denied in database 'master'
which comes from below line of code:
Dim user As ApplicationUser = manager.Find(UserName.Text, Password.Text) I wonder why it connects me to master database when I have set to another specific database name. In my web.config, I have set to 
<add name="DbConn" connectionString="Server=.\SQLExpress2;AttachDbFilename=C:\Program Files (x86)\Microsoft SQL Server\MSSQL12.SQLEXPRESS2\MSSQL\DATA\UMS.mdf;Database=UMS;Trusted_Connection=Yes;" providerName="System.Data.SqlClient" /> And in my Identity Mod ...