Steps to reproduce:

1. Download and import attached project. (repro_case_761588-Battle_Jump.zip)
2. Load scene: Menu
3. Inspect error messages in console.
4. Try to build the project to standalone

Expected result: project builds, no errors are visible.
Actual result: project does not build, error messages appear:

UNetWeaver error: Exception :System.ArgumentException: Member 'System.Int32 UnityEngine.Networking.NetworkConnection::hostId' is declared in another module and needs to be imported
at Mono.Cecil.MetadataBuilder.LookupToken (IMetadataTokenProvider provider) [0x00000] in <filename unknown>:0

Failure generating network code.
UnityEditor.Scripting.Serialization.Weaver:WeaveUnetFromEditor(String, String, String, String, Boolean)

And this one after build:

error CS0016: Could not write to file `Temp/Assembly-CSharp.dll', cause: Sharing violation on path C:\Users\Justas\Downloads\repro_case_761588-Battle_Jump\Temp\Assembly-CSharp.dll.mdb

Reproduced on: 5.4.0b19, 5.3.4f1, 5.2.4f1