Failed to enter map

Development Programming & Scripting
1

I am using FPlatformProcess::CreateProc to launch a game instance but the new process fails to enter the map.
The params used are simply

FString ExePath = FString::Printf(TEXT("App.exe /Game/Maps/Default?Game=Base -server -log"));
FPlatformProcess::CreateProc(*ExecPath, nullptr, true, false, false, NULL, 0, NULL, NULL);

The log reports

LogPackageName: SearchForPackageOnDisk took   0.596s, but failed to resolve (null).umap.
LogLoad:Error: Failed to enter /Game/Maps/Default: . Please check the log for errors.

Using the params with command-line works and the map is packaged. Solutions?

p.s. Base is set in the config, it is an AGameMode based class.