Everytime you start the XAS it will create an XASInstance object (see the metaobject XASInstance in the System module in the Modeler). This object is used to manage multiple XASInstances if you are running multiple XAS instances in combination with a load balancer.
One of the XAS instances is cluster manager. From time to time this manager will check what XAS instances are alive. If an XAS Instance isn't alive anymore its associated object will be removed, the log line you state in your answer will be logged. If the cluster manager itself isn't alive anymore one of the other instances will pick up the manager role.
If you are developing you re-start your XAS a lot of times. Everytime you re-start the old XAS instance object (from the previous time you did run the XAS) will be detected and removed because it isn't alive anymore.
This means an XAS instance which is registered in your database, has not updated itself within a certain interval (default ten minutes).
A situation in which you can get this message is when you start your project in Eclipse, but do not terminate the server by entering 'x' in its menu, but by clicking the red 'Terminate'-button instead. This button causes the application to not terminate neatly, which causes the XAS-instance entry to remain in the database (normally it would be removed). If you restart the application later on, the XAS-instance entry will still exist in the database, but in fact does not exist anymore, which will cause the timeout and accompanying log message.
It might be that the XAS was automatically shut down at random. Do you have a licence key?