Hey! I had the same issue and it was due to the thumbnails not being correctly saved. I was able to fix it enabling "Save Thumbnail" in Preferences/General/SaveLoad. The only problem will be that your old files will not be able to be recovered, maybe by opening them from a windows computer if you have one at hand.
If after enabling the "Save Thumbnail" option the error still appears, make sure you delete (or move) your old files from the last saved directory as PixelComposer will have this path stored as the last saved location and will still trigger the problem when you try to save or open files. Hope it helps!
