What does error 422 mean in Minecraft modpacks?

Error 422, often referred to as 'Unprocessable Entity,' in the context of Minecraft modpacks typically indicates that the server is rejecting a request because the data provided (e.g., mod configurations, player data) is invalid or doesn't conform to the expected format. This usually points to a problem with a mod's configuration, an outdated mod, or an incompatibility between mods.