The condition must be inverted here...

--- UI/WebServerResources/ContactsUI.js 55fb0b5897aa17513c64ddb051495d11d8cc1f75
+++ UI/WebServerResources/ContactsUI.js ccd979e28b580fc6d6520731e3beaef4e0a77f31
@@ -527,6 +527,8 @@ function newList(sender) {
var li = $(Contact.currentAddressBook);
if (!li.hasClassName ("remote"))
openContactWindow(URLForFolderID(Contact.currentAddressBook) + "/newlist");

• else
• alert (clabels["You cannot create a list in a shared address book."]);
  return false;
  }