When you have the IggyScrotch Phoenix mesh in your level, you likely get this error message when upgrading to 4.25.
[2020.05.19-14.10.48:464] 0]LogWindows: Error: === Critical error: ===
[2020.05.19-14.10.48:464] 0]LogWindows: Error:
[2020.05.19-14.10.48:464] 0]LogWindows: Error: Assertion failed: !Obj->HasAnyFlags(RF_NeedLoad|RF_NeedPostLoad|RF_ClassDefaultObject) || bIsOwnedByCDOOrArchetype [File:D:/Build/++UE4+Licensee/Sync/Engine/Source/Runtime/CoreUObject/Private/UObject/UObjectGlobals.cpp] [Line: 2396]
[2020.05.19-14.10.48:464] 0]LogWindows: Error: Attempting to replace an object that hasn't been fully loaded: ClothConfigNv /Game/Paragon/Characters/Heroes/IggyScorch/Skins/Phoenix/Meshes/IggyScorch_Phoenix.IggyScorch_Phoenix:Pyro_Phoenix_APEX.ClothConfigNv (Outer=ClothingAssetCommon /Game/Paragon/Characters/Heroes/IggyScorch/Skins/Phoenix/Meshes/IggyScorch_Phoenix.IggyScorch_Phoenix:Pyro_Phoenix_APEX, Flags=0x002c3008)
[2020.05.19-14.10.48:464] 0]LogWindows: Error:
Removing the Apex mesh data fixes this critical error.