From your log, you can see that you have a reference to a Seat object that needs to be cleaned.
[2015.02.18-16.07.44:362][226]LogReferenceChain: (root) GCObjectReferencer /Engine/Transient.GCObjectReferencer_0->UE4Editor-CoreUObject.dll!UnknownFunction (0x000007fef2b33770) + 0 bytes [UnknownFile:0]
[2015.02.18-16.07.44:362][226]LogReferenceChain: Scene2_C /Engine/Transient.UnrealEdEngine_0:GameInstance_0.Scene2_C_0->Seat
[2015.02.18-16.07.44:362][226]LogReferenceChain: StaticMeshActor /Game/StarterContent/UEDPIE_0_Scene2.Scene2:PersistentLevel.Seat2->Outer
[2015.02.18-16.07.44:362][226]LogReferenceChain: Level /Game/StarterContent/UEDPIE_0_Scene2.Scene2:PersistentLevel->OwningWorld
[2015.02.18-16.07.44:362][226]LogReferenceChain: (target) World /Game/StarterContent/UEDPIE_0_Scene2.Scene2
[2015.02.18-16.07.44:433][226]LogLoad: (Object is not currently rooted)
So that seat reference need to be cleaned. You can put it to null or detraoy or both before switching level. This happens because seat is part of the world that needs to be cleaned.