Just make sure you re-encrypt the app.dat, you don't need to rehash/resign. Try restoring from the backup and try again. Also depends on the hex editor you use, too. I was using Hexecute first, but I had problems with that (including one time writing a paste over the header at the beginning without me noticing it). HxD works a lot better for me. Just be careful, save editing is unforgiving, any little change done wrong will blow up in your face.
Work in the hex field whenever possible, since multiple hex values can have the same appearance in ASCII. If worse comes to worse, cherry pick which items you want and recreate it afte zeroing out the original. I did this by skipping a few lines down and using that for a temp area, carefully creating a list with the right ids, the right number of zeros, and the proper value to make each entry 18 bytes, and when I had the whole list, I copied it (including a couple rows of zeroes after it, clicked on the first byte of the item list (making sure not to change the previous number at all like I said before), and pasted over it, then zeroed over anything that remained in the old list, and the temp area I copied from.
At first, I got a lot of corrupted saves, too, since I was just getting the hang of it, and also dealing with tools that weren't working right. Once I started using Modio 4 and UxD, I had gotten used to making several edits, so I haven't had a problem since. Practice makes perfect.