I'm using batch actions to export a collection of Freehand (fh11) files to various other formats (emf, swf, etc.). It's a simple process - load the freehand file, export it to the desired format, close the image. Everything works swimmingly for the first 454 files (give or take a few), after which I get the message that "the file 'whatever.fh11' is in an unknown format and cannot be opened". The batch stops and I can't even manually open this or any other valid fh11 file. The indicated fh11 file is indeed valid - once I shut down Illustrator I have no problem loading it.

I've thus far converted 20,000 or so fh11 files using batch actions and have had to restart Illustrator every 454 files during this process. Anyone have any ideas?

Without knowing more about the files, I would recommend to just reduce the number of files you're processing in a single Batch process.

Sometimes, especially if there are too many files, Illustrator's Batch process gets confused and strange things may happen then.