To reproduce:

1.Download and open attached project
2.Build the game on standalone platform
3.Open built project and play as Host
4.Enter play mode in the editor
5.Connect to hosted server as client from editor

Reproduced with: 5.5.0f3, 5.6.0b3, 5.4.3p3, 5.3.7p2

Expected result: Second player connecting succesfully to the hosted server and editor not throwing error.
Actual result: Second player does not connect to the server, editor throws error.

Note: Also reproducible when server is hosted from the editor and the client connects from the built game.

Verified with: 2017.3.0a2