Not sure why you're seeing that unless you enabled "Include Message" in the notification settings. Even then it should be replaced with the actual message. Did you add [message] with a language override or maybe an out of date language plugin? There are two separate language strings for the notification for replies and new messages. They are as follows.
Replies
Key: FROM_HAS_REPLIED_MESSAGE_WITH
String: [from] has replied to your private message.<br /><br />[message]
Key: FROM_HAS_REPLIED_MESSAGE
String: [from] has replied to your private message.
New
Key: FROM_HAS_SENT_MESSAGE_WITH
String: [from] has sent you a new private message.<br /><br />[message]
Key: FROM_HAS_SENT_MESSAGE
String: [from] has sent you a new private message.