Random dbnetlib connectionopen (connect()). sql server does not exist or access denied

random I receive in windows server 2012 the following errors: "dbnetlib connectionopen (connect()). sql server does not exist or access denied".. It seems to appear after some connections. Any ideas? I have just checked port on firewall, connection strings, and so on

Context. W does your environment look like. Is this error from ssms, a web site, ???

It ' s from a visual basic ERP

please post the vb connection code here. is this a new app or old , a new sql server or existing? you need to provide more details.

It a sql server 2012 which host several other databases..All run correctly, only one has this problem.