Thanks dann for your helpful file. You'll be able to open your file again with the version 5.3 Beta 11 and further.

This new Beta version fixes also two other bugs:
- Texture image files exported with OBJ file are copies of original image files instead of files generated from images in memory when available (see bug #686)
- The area of rooms are correctly computed when points draw a hole in its middle (see bug #681)