Hi Vassilis,
There are a few SAP Notes on this issue I found:
http://service.sap.com/sap/support/notes/2192737
http://service.sap.com/sap/support/notes/1873607
http://service.sap.com/sap/support/notes/1873559
They look as though they can provide some good advice.
Kind Regards,
Nick Lakasas