I am calling
GetWorld()->ServerTravel(travelURL,false,true);
from within the GameMode,
but it disconnects all other Clients.
I already tried using Standalone game but that didn’t help.
I am using the Null OnlineSubSystem (LAN).
Any way to fix this?
Desperate Self-bump 
And now I also found the solution thanks to someone on Discord.
The console Command (per hand) [FONT=courier new]servertravel mapname worked and [FONT=courier new]UWorld::ServerTravel did not.
Even[FONT=courier new] UWorld::Exec() did not work with the servertravel command, nor [FONT=courier new]APlayerController::ConsoleCommand()
I investigated further and found the correct function to Call:
[FONT=courier new]AGameModeBase::ProcessServerTravel().
Everything works now how it should work.
€dit:
Someone had the idea that the last parameter of UWorld::ServerTravel is ambigous.
The description of the parameter is as follows:
However, the parameter itself is called
[FONT=courier new]bShouldSkipGameNotify
This is a contradiction. When reading the description (what you are reading when filling in the parameters in VS) i expect to type [FONT=courier new]true, although the correct input would be [FONT=courier new]false.
Epic, pls Fix. @Amanda.Bott
Thank you so much stuck on this for the past day wondering why my clients were getting disconnected when the server was travelling.
I can confirm that as of 2019, UWorld::ServerTravel() still does not work and AGameModeBase::ProcessServerTravel() is still the way to go (for anyone coming back to this thread).
With things not working, I’ve been wracking my brain wondering if I just didn’t understand Unreal, but then it just turns out to be a bug. 
Edit:
And I found my error. I guess I just don’t understand Unreal as well as I though yet. 