When you have Discoverer and EBS running on the same machine, this kind of conflicts may happen. To resolve, Start another session, clear All Environment Variables, restart the Web-logic Server Again. This will resolve the issue. For More Info, Refer
| Unable to Access Discoverer Plus/Viewer 11g Installed On Server Having EBS 11i / R12 (Doc ID 1338244.1) |


When you have Discoverer and EBS running on the same machine, this kind of conflicts may happen.
To resolve,
Start another session, clear All Environment Variables, restart the Web-logic Server Again.
This will resolve the issue. For More Info, Refer