The encryption token challenge/response portion of the control channel handshake can fail due to a seamless travel occurring between the client's NMT_Hello being received by the server and the server finishing its key lookup and calling the encryption response delegate. This is due to the delegate being bound to the UWorld (even though the lookup logic occurs on the game instance), which will cause the object delegate to become unbound when we GC the previous world during the travel.
Requires a client to connect to a server, send NMT_Hello with an encryption token, and then for the server to seamless travel to a new map/world before the encryption response is received.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-81365 in the post.