Hi, I eventually tracked down the source of the problem to a simple piece of geometry (literally a poly box serving as a garden bed, imported from Revit originally) that had a Forestpack object applied to it. Loading the scene with 5.4.0, deleting the object, saving, installing 6.1.1 and then recreating it and re-applying the Forestpack object in 6.1.1 works fine.
I'll send the 5.4.0 scene with the original offending object when I have a bit of time.
Cheers