Lion: Keynote error "Couldn't save image XYZ" while duplicating file

I am currently writing a major presentation (about 30 MB, 125 slides) using Keynote under Lion - and my heart just stopped for a moment:

I do regular backups of the document I'm working on using Keynotes "Duplicate" function and then saving the copy under a new file name.

Suddenly while "Saving" before "Duplicating" I get loads of "Couldn't save image XYZ" errors and all images in the duplicated file have been replaced by placeholders ⚠

What is going on? Is there a maximum file size in Keynote?

Or a maximum number of images it can handle (my presentation is quite heavy on images.) I already used the "Reduce file size" option.

Any insight would be appreciated.