Crash on server when a client joins the session

We're trying to update our projects to Unreal 4.18.2, in order to support Oculus Dash within our app.
The same app worked with UE 4.16.3 but crashes with this new version of the engine. It crashes on server side as soon as a client tries to join the server's session-

The app crashes with this error message:

Assertion failed: NumberOfFreeBits >= 0
[File:D:\UnrealEngine-4.18.2-release\Engine\Source\Runtime\Engine\Private\NetConnection.cpp] [Line: 1431]

The engine is the one of Epic, version 4.18.2, downloaded from github and built on our side. As it happened in the past, it works correctly with the Null Online Subsystem, but crashes when using the Oculus's one. The same happened with the version 4.18.1 (don't know if also with 4.18.0).

May you investigate on what is the cause of the error?