Hi Riza,
We are having the same error here.
We are running a windows server VM with 4 cores, 16 GB of ram. The server was very stable with a small number of services on it. We now have roughly 250 services published and we are getting this same error.
SEVERE May 30, 2013 9:57:22 AM Unable to process request. Error handling service request : Cannot obtain a free instance.; nested exception is: com.esri.arcgis.discovery.ejb.ArcGISServerEJBException: java.lang.Exception: Could not initialize service '8200_to_8299.t8288_Routing.MapServer'. Rest
SEVERE May 30, 2013 9:57:22 AM Could not initialize service '8200_to_8299.t8288_Routing.MapServer'. Server
SEVERE May 30, 2013 9:57:21 AM Unable to process request. Error handling service request : Cannot obtain a free instance.; nested exception is: com.esri.arcgis.discovery.ejb.ArcGISServerEJBException: java.lang.Exception: Could not initialize service '8200_to_8299.t8288_Routing.MapServer'. Rest
SEVERE May 30, 2013 9:57:21 AM Could not initialize service '8200_to_8299.t8288_Routing.MapServer'. Server
SEVERE May 30, 2013 9:57:21 AM Unable to process request. Error handling service request : Cannot obtain a free instance.; nested exception is: com.esri.arcgis.discovery.ejb.ArcGISServerEJBException: java.lang.Exception: Could not initialize service '8200_to_8299.t8288_Wetlands.MapServer'.:
We don't know what the cause of this issue is.
Brett