When using a cubemaps assigned to a Post Process Volume and loading the level the editor will freeze if it's been a project converted from 4.11 to 4.12/4/13. In some instances if the camera is outside of the PPV and you move the camera back into it the editor will freeze at that point.
Release 4.12.2 - CL-3005592
Dev-Main - CL-3007689
Head over to the existingAnswerHub thread and let us know what's up.