Shoutbox

The text it is replace with, shouldn't be longer than the original text...
(Not the case here, I think)
Just use:
function OnEvent_ChatWndReceiveMessage(ChatWnd, Origin, Message, MessageKind) {
   var OtherMessage = "abc"
   return OtherMessage
}

If it doesn't work, try posting the code here...

-edit- and some minor suggestion, try to keep you scriptcommands together using a small prefix, something like denKey and denAutoEncrypt
It'll show up in a group in the ScriptCommands window