"An Internal Error occured whilst opening the Diagram"

Prior to version 11.1.1.6, this error occurs when you click on the mapping tab of an interface.
"An Internal Error occured whilst opening the Diagram"
Restoring an old backup wont work. The work around for this as suggested by Oracle (Note 1279132.1) is given as follows. 

1. Find  the internal ID of the Interface (click on the Version Tab of Interface)
2. Execute the query on the Work Repository

delete from SNP_TXT

where I_TXT in (
                 select I_TXT_XMI_DATA
                 from   SNP_DIAGRAM
                 where  I_POP = InternalID;

delete from SNP_DIAGRAM
where where I_POP = InternalID;

commit;

Best of Luck !!!