Server Travel Crushes the engine. How to use it right?

Hi guys.
Ive developed simple multiplayer game and begin to create Lobby - Game transition after all other things become done. And heres appeared a major problem. I was created a simple session create / session join statement into my lobby map and its seems to work good, but on the transition part whole engine is crushing. Im already googled it, try different patterns of fixing, but result allways the same. I using v.4.16.2 at first and then update to 4.17, several times created a basic project with two maps - “Lobby” and “Main”. Then on keypress “Tab” on server callis the “ServerTravel Main” (they all places into root content folder), maps are added to list into project settings. Then change the players count to 2 in engine settings and press tab on server player. Then sever transitting to “Main” map, and the client stay in “Lobby”. But they are still connected, cause all the printstrings are displaying at both players screens. So heres the question: how to use server travel right, or maybe there is any good tricks to avoid its using. Thanks.