Hi Orbital. 
Lately that problem repeats with a certain constancy with me, what left me deeply sad with Bryce. 
 
I noticed (in my case) that is linked to countless changes of textures. 
Even after changing the texture of an object she is in the relationship of the editor of textures of Bryce, a great mistake. 
Another thing that I noticed is that to each new mistake, the file increases your size. 
I tested in XP 32 and 64, in Vista 32 and 64 and it didn't change a lot of thing, the mistakes continue. 
 
My entrance in the challenge was reduced originally of 340 MB for 148, what decreased in a lot the result in the final quality. 
After many days I got to recover the file saving him/it with other name, but that is not rule, most of the time the file is lost. 
 
I hope gets to solve and inform us. 
Hugs.