I’ve got a bit further and worked out that it was the Steam sockets plugin that was causing the “Error creating network driver” issue. I’ve now swapped to using the IP net driver, and I can connect clients to the server if use the option for the Unreal editor to automatically spawn and connect clients when running Play In Editor (see Editor Preferences > Level Editor - Play > Multiplayer Options).
However, looking more closely at the server logs in this case, it appears that the Online Subsystem Steam actually gets disabled:
[2020.12.07-09.06.07:735] 0]LogOnline: OSS: Creating online subsystem instance for: STEAM
[2020.12.07-09.06.07:736] 0]LogOnline: Warning: STEAM: Steam API disabled!
[2020.12.07-09.06.07:736] 0]LogOnline: Display: STEAM: OnlineSubsystemSteam::Shutdown()
Packaging the project with Steam sockets disabled does not cause these messages to be printed when the server is run, but of couse clients can still not connect to a packaged dedicated server.