A recent Microsoft Security Update (KB928365) seems to be causing issues for some Managed applications when they try to access a data source (like SQL Server 2005). This update is designed to fix MS07-040, which is rated as a critical vulnerability. After installing KB928365, many people are seeing intermittent errors like you see below.
The error looks like this:
System.InvalidOperationException: Internal .Net Framework Data Provider error 6.
at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
Uninstalling KB928365 seems to resolve the errors.