The Illegal deck error means it breaks a deck building rule. (more than 8 climaxes, more than 4 cards with the same name, having more or less than 50 cards) If the formatting was incorrect the error would be "Error loading deck: Card not Found"
Note it is only intended to read files created by this program, you are not meant to modify these files manually. The card ids used internally will not always match the card id written on the card or used on other websites.