Login failed - not associated with a trusted SQL Server connection

I'm getting the following error: login failed for user '(null)', Reason: Not associated with a trusted SQL Server connection.

Here's my connection string :

<add key="ConnectionString" value="Persist Security Info=True; Trusted_Connection=Yes; Integrated Security=SSPI; Application Name=CCMIS; Initial Catalog=CCMIS; Data Source=NameofServer; Network library=dbmssocn; Packet Size=4096;" />

I'm running a windows application that calls a web service to update a SQL database.

The application is using a windows group account and the between the web service & the SQL server it's using a domain account. My user-id is a member of the windows group account. I'm not sure of the relationship between the domain account and the group account or if there is one? I'm new to this permissions stuff.

The DBA tells me nothing has changed on the SQL side and I haven't done anything to the application (which was working yesterday). Any ideas?

The error message indicates the login passed in the connection string (that's the Windows login account in this case) is not recognized by the SQL Server. So you need to ask your DBA to add the Windows login account on your client server to the SQL Server logins.