The full error message will look like the following:
If you check the logfiles (from the sending client) you will then see the following:
(The request was rejected because the application did not respond to the incoming call and it was not the preferred endpoint)
This issue could happen due to the reason that a SfB user use multiple SfB clients at the same time. However IM/chat will only go to one endpoint called the preferred endpoint. This is calculated internally within SfB using an algorithm which determines which client is most active and recently available. It can get this wrong if a user has a number of devices logged in. The preferred way is to use only one client at the same time.