Dears,
Just to update you .
I have got similar problem and solution to that was oracle account using for DP integration didnt have sysdba prvillege . The was the reason using oracle user name and password in SQL promt worked but util_oracle8.pl failed with same credntial, because util_oracle8.pl uses user names as sysdba when connecting.
summary
1) Make sure DP integration oracle account has sydba prvilege ( from 12C I beleive Oracle introduced sysbackup privillege to avoid granting sysdba for backup)
2)Double check Oracle_Home , Oracle_SID
Sincere Regards,
Aneesh