Hero’s Mod is trying to register the same hotkey/action twice, probably during multiplayer/server world loading. This is a known issue in mods that aren't updated properly for newer tModLoader versions or conflict with other mods.
Exactly — all signs point to Hero’s Mod being the cause:
✔️ Works fine without it
❌ Throws ArgumentException: An item with the same key has already been added when enabled
🔁 Reinstalling/resetting config doesn't help → means it's likely a mod code issue, not just a bad config
it only happens when i try to start a server with a world ok
can you check that ok