Hi,
What is the java version that you are using in NWDS.
https://nwds.sap.com/swdc/downloads/updates/netweaver/nwds/nw/731/doc/files/sapjvm6.msi
Can you please check the .log file in the .metadata folder of your workspace.
Check also the proxy configuration (if you use any) in NWDS' preferences.
You find it at "Window -- Preferences -- General -- Network Connections.
Can you access the NW 7.4 server from this NWDS server.
Can you add any other system?
Thanks
Rishi Abrol