I found a solution to this problem, so I'm leaving a note for anyone who is having the same problem.
The default peds.ymt (file size around 240) is encrypted, and if you open it in edit mode and save it, the game will refuse to start. To start the game normally, you need a decrypted version of peds.ymt (file size around 2400).
I couldn't find a way to decrypt it, but as long as the file itself is decrypted, I looked for a MOD that provided a decrypted peds.ymt and used that file. Copy all the contents of the encrypted peds.ymt you have, overwrite the contents of the decrypted file, change the parts you want to edit, save it, and finally replace peds.ymt from the encrypted one with the decrypted one, and you're done.
The decrypted peds.ymt file extracted from the MOD is edited for that MOD, so don't forget to overwrite the contents with the default contents.