What if the 'Minecraft not a map' error occurs after updating Minecraft or a modpack?

If the error appears after an update, the update might have introduced an incompatibility or corrupted existing world data. For vanilla Minecraft, try verifying the game files. For modpacks, it's often best to try loading the world in a previous version of the pack or consult the modpack developer for known issues and solutions. Sometimes, updating or downgrading specific mods can also resolve conflicts.