The error in Age of Empires II: Definitive Edition is a false positive in many cases, triggered by legitimate software (overlays, mods, antivirus). If troubleshooting fails, the most reliable solution is:
If you have ever opened the key-value-modstrings.json or player.nat file in Notepad to tweak zoom levels or network settings, you may have inadvertently introduced an invalid character. The game checks hash values; even changing a "0" to a "1" can break integrity.