Textures messed up and lighting build fails.

Closed file yesterday - with everything looking good. Open file today and white has become black and shadows are a mess. Reopened file several times, and a small number of the textures have been re-established. Attempted lighting rebuild several times, but it lasts about 10 seconds before failing and giving the message, “Lightmass crashed. [FONT=“Helvetica Neue”]Assertion failed: (Index >= 0) & (Index < ArrayNum) Array index out of bounds: 3775 from an array of size 3775.”
I would not consider this to be a large file - it is only the externals of a single domestic dwelling with virtually no interior geometry, I can’t imagine memory is a problem have 16gb of RAM and running a 1080gtxti. The problem occurred at start up, so I can’t imagine there is a cache issue.
Is this down to Datasmith - as I haven’t experienced these issues previously with fbx imports? Repeated crashes are very frustrating when needing to supply work to a client.
What possible solutions are there for eliminating this issue?