Weird. I've not seen this error before, thanks for reporting! There's a lot of extra stuff that's just Pokemon Essentials data but not used in the demo. The Unown sprites never appear so the game may still run fine. Does it still run without those files? Or, have you tried extracting all but those files?
This is good to know, none-the-less, as I intend on purging a lot of the extra stuff in the final product (which, will probably prevent errors like this). But, I don't know all the gags I'm going to pull at this point, so I'm waiting to the final product before removing all that crap.