Host name is invalid or not reachable

Host name is invalid or not reachable

I am trying to install OpManager 9.0 but at the point of configuring the MS SQL server details it always fails with the following message:
 
"Host name is invalid or not reachable"
 
I've specified the details as follows:
 
Hostname: mysqlserver\opmanager
Port: 50490
Database Name: OpManagerDB
Username: mydomain\opmanservice
Password: ********
 
I know these details are correct but still it fails.  Any suggestions for getting past this?  It's not a firewall issue and I've given the user rights on the SQL server.
 
We currently use OpManager 8 with SQL Express on the same box but want a fresh install on a new server using SQL on a different server.
 
Thanks,
Dave

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