@Ashok Kumar 
you prompt reply would be really appreciated.

@Ashok Kumar  Thanks for sharing the details. could you please share the further granular
level mapping that maps individual source with Target such as Properties in the Medication class to HS.SDA3.Medication (property level mapping in the DTL. I completely understood you mapping at source level but i need to do each source level mapping in the following DTLs.
source.diagnosis to HS.SDA3.Diagnosis
source.Immunization to HS.SDA3.Vaccination 
source.Medication to HS.SDA3.Medication

@Eduard Lebedyuk  do you think this would the cause of this error? What i am thinking is the DB connection open and Close is not in sync. It tries to open the DB connection while it is not closed properly or not even closed.

Is there a way i can check the connection is open or not before initiating/opening it again or establishing the connection with an external DB?.

@Luis Angel Pérez Ramos
When you say "My suggestion, try to get the schema of the CCDA (a XSD file)" do you mean by CCDA.XML file? if no then how i can get the aXSD file. I have only CCDA.xml file. Or can i generate this XSD file using the CCDA.xml file?