Help with connection string

I have a connection string in my web.config file:

<add key="cnString" value="data source=ServerName;initial catalog=DatabaseName;password=Password;persist security info=True;user id=UserName;packet size=4096" />

and it works just fine with the SQL production server.
Recently the testing SQL server was created and I can login to it with Enterprise Manager.

But when I try to change the connection string just putting a new server name, database, user name, and password, it gives me the error:

Login failed for user 'UserName'

What should I look in the database or SQL server that is different from a production one? Or what should I ask since I don't have admin rights to both servers?

BTW, I've already tried to play with Trusted_Connection parameter (True/False) and it didn't help.

Thank you in advance for your help.You should check whether the user you are specifying in the connection string exists on the sql server and whether that user has access to the database that you are setting as "Initial catalog"

Hope that helps
Kashif|||Yes, the user exists and has an access to that database.