Frame buffer 0 invalid

[Christian Götzinger <cg(at)pension-goetzinger.de>]

Hey guys,

I have this scene that I need rendered asap. It's a very big scene and 
I'm happy that it rendered properly. Well, it rendered properly, and 
when it was time to write the output file to disk, Mental Ray chose not to:
Error 051003: frame buffer 0 invalid, cannot create file

So, after 27 hours of rendering I'm sitting here without a file and have 
no clue what went wrong. I had rendered out a small preview (800 pixels 
wide) of the very same scene just a few days ago. The only thing that's 
changed since then are a couple of material settings, higher render 
settings and a higher output resolution (3000 pixels wide).

I need this darn scene rendered, what can I do? I mean what on earth did 
Mental Ray do with all the data it calculated over the last day? Threw 
it away?

Cheers,
Chris