I know this is old but something important to watchout for when testing in editor is always launching the play-session with “Play-Standalone” selected and not “Play As Listen Server”. ← This also applies for "standalone sessions”
If FE: Play as a Listen server is selected you already launch into the menu as a server and your selected port is instantly occupied, now when trying to create a new session it is unable to link to the same port and fails to listen correctly.
On a side note the events: OnNetworkError and OnTravelError always help to troubleshoot potential issues.