I have an instance that is unreachable in SQL Safe using the SERVERNAME\INSTANCE but if we use IP Address\INSTANCE to connect from the management console it works fine. DNS has been ruled out as the issue because SERVERNAME\INSTANCE works fine in SSMS and every other connection method.
The SQLSafe error logs show the following error.
'SQL Server (unknown version), [-1] A network-related or instance-specific error occurred while 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)'
The other thing to mention is that there is a second instance on the same server that can be reached using SERVERNAME\INSTANCE in SQLSafe.