Getting the same issue.
Blueprint
All level transitions worked flawlessly in 10.2. Just upgraded to 10.3 and level transitions crash the game pretty much every time. I even made a basic level out of the starter content third person level and added an overlap actor that triggers another generic level load. Crashes.
Callstack:
UE4Editor_Core!FDebug::AssertFailed() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\misc\outputdevice.cpp:374]
UE4Editor_Engine!UEngine::LoadMap() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\unrealengine.cpp:9480]
UE4Editor_Engine!UEngine::Browse() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\unrealengine.cpp:8682]
UE4Editor_Engine!UEngine::TickWorldTravel() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\engine\private\unrealengine.cpp:8857]
UE4Editor_UnrealEd!UEditorEngine::Tick() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\editor\unrealed\private\editorengine.cpp:1285]
UE4Editor_UnrealEd!UUnrealEdEngine::Tick() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\editor\unrealed\private\unrealedengine.cpp:361]
UE4Editor!FEngineLoop::Tick() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\launch\private\launchengineloop.cpp:2427]
UE4Editor!GuardedMain() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\launch\private\launch.cpp:142]
UE4Editor!GuardedMainWrapper() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:264]
kernel32
ntdll