If the error happens or only when joining a multiplayer match ?
AoE2: DE receives frequent updates (new civilizations, balance changes, bug fixes). Sometimes a patch fails to write correctly. You might have a mix of old and new DLL files. The game checks the hash of these files against a server-side list. If they don't match—even due to a benign download hiccup—the error appears. age of empires 2 definitive edition tampering detected
If you have tried all the steps above and still encounter the error, you may need to perform a clean uninstallation, delete the leftover files in your Steamapps/common or AppData folders, and reinstall the game completely. To help narrow this down, please let me know: If the error happens or only when joining
Add both the AoE2DE_s.exe (the game executable) and your entire Age of Empires 2 installation folder to the exclusion list. You might have a mix of old and new DLL files