I just fixed this for myself, same exact issue. Browse to the DayZ game installation folder and find the folder marked "dta" and "addon". Inside those look for any '.pbo' files that have a different date than the one from the game installation and delete them. I had the game installed for nearly three years my files were easy to spot. Hope this helps.
Edit: Another way is to delete the 'dta' and 'addon' folder completely then have steam verify file integrity to download the missing pieces.