IM not sent to user "This message wasn't sent to...."

One of the common reasons for this problem is that the recipient is logged on to both the desktop and the mobile client. It is very important that you are not logged in to any mobile device when logged in on your desktop client.

So if you get this message when sending IM to one or more of your contacts, ask them to log out of the mobile app.

PS! Remember that you have to actually log out from your mobile app, you are still logged in if you just close the app.