SQL error when connecting to server using servername\instance.
Error:
A network-related or instance-specific error occurred when establishing a connection to SQL server. the server was not found or was not accessible.Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error:26 - Error Locating Server/Instance Specified) (Microsoft SQL Server)
Solution:
The UDP port 1434 on destination server was being blocked in the firewall.