It's not a hack. The error is showed and you still can't get it. Your mu.exe is altered. Download the latest patch by clicking
here then extract all files and folders in root client folder of InfinityMU Client, it will ask to replace with old files so you click yes to all.
This error appears usually to users who have made changes on our files which is not allowed. Also appears if your auto-update launcher failed to download latest modifications from our server. Just extract latest patch over client and you will not have problem to connect.