Here is a 3rd party link which discuss it further.
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The behavior is usually a combination of the following:
- SQL Server Browser Service isn’t running
- IP is not configured correctly in the TCP/IP settings (Navigate to SQL Server Configuration Manager, under the Client Protocols right click on TCP/IP and click Properties)
- Port 1433 (default port for SQL communication) isn’t enabled on the IP or is being blocked by a firewall (could be on SQL server or on REWS server. Could be network firewall or Windows firewall on one of the servers)