How to reproduce:
1. Open the attached project
2. Make sure that the Scripting Backend is set to IL2CPP and the Architecture is x86_64
3. In the Build Settings, enable Development Build and Script Debugging options
4. Build the project for the Standalone platform
5. Launch the Build
6. Click the "Start Server" button once the build launches

Expected result: Server starts without any exception.
Actual result: A "ENetException: Host creation call failed" exception is thrown.

Reproducible with - 2018.1.9f2, 2018.2.8f1, 2018.3.0b1, 2019.1.0a1

Note: Not tested on versions prior 2018.1, since support for IL2CPP scripting backend for Windows Standalone was not yet introduced.

Update: The issue is by design. This was a marshaling bug in the user code.