Hey Simon,
I get heaps of different error messages every now and then
but haven't heard of that one specifically.
Have you tried opening the back-up version and then if you are able to open it save it as a pln file and work from that one instead?
Or if its the third time this has happen...each of the 3 times did you do the same thing, for example put in a million trees and save and then try and open it again, I'm not saying its 'the trees' but the problem might be occurring because of one of the steps you are doing prior to saving.
Adrianne