Open level crashes editor

My game has a persistent level that holds many streamed levels. The game begins however, in another level (no sublevels). The game packages and plays this opening level fine (4.17.2 - Windows 64bit).

My issue is I can’t open this starting level in the editor - it crashes. It opens in 4.16.3
I have tried trashing the: Saved, Intermediate and WindowsNoEd folders. still crashes.

ideas please?