Make sure that there is no other instance of the same project is open. In my case (which is still happening every now and then), I realised that when I am running unreal or the project, I am triple of quadriple clicking instead of double clicking when trying to open the project, which leads to another instance of unreal running. Closing one of the 2 instances, usually solves it.
Another think you might want to look in to is basically making sure that the asset is not open somewhere else e.g. and FBX file that is also opened in blender or Maya …etc