The user instance login flag is not supported on this version of SQL Server. The connection will be closed.

I got this error this morning when moving from a SQL Express database to SQL Server 2005. The fix was simple: I just needed to remove the "User Instance=True;" in the LocalSqlServer connection string, from:

<add name="LocalSqlServer"
connectionString="Data Source=(local);
Integrated Security=True;User Instance=True;
Initial Catalog=dbname"
providerName="System.Data.SqlClient"/>

to

<add name="LocalSqlServer"
connectionString="Data Source=(local);
Integrated Security=True;Initial Catalog=dbname"
providerName="System.Data.SqlClient"/>