A network-related or instance-specific error occurred while establishing a connection to SQL Server

I have SQL Express 2012 SP1 x64 installed on one server (a totally default install), and from two other servers that have the SQL Native Client installed (32 or 64 I don't know as it installed as part of Citrix software), and from these two servers when I try to connect to the SQL server, I get:
“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: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
  From one client I can ping the SQL server by both name and IP.  The SQL server has two SQL services running, SQL Server and VSS Writer.  I started SQL Server Browser (since it's description says it provides connection info to clients) and restarted SQL Server but that didn't h ...