This is something I shared quite some time ago (back then it was APEX 4.x, but it's still valid) on another forum. But I got the question back lately. So, here I am back with the same message ....
An integrated Apex App with Oracle E-Business Suite (EBS) has an unfortunate side effect concerning Interactive Reports.
Some of the settings of such a report is easiest done when running that report as a developer and then save it as Default and Primary Report Setting. So everybody who runs the report gets to see it the way you - as developer - has saved it.
When you execute the Apex app from within the EBS, the user you're using to connect to Apex is inherrited from EBS and as such not an Apex Developer user. Consequence: you are not allowed to save your IR as Default setting!
The trick around this to logon to Apex builder (as developer obviously) and to the EBS with the same browser session. Then launch your app via EBS. So the app opens in the same browser as the one used by your Apex builder.
In your app you have now the oportunity to use the edit-bar (at the bottum: "Home" / "Application" / "Edit page" / .....) and ... most of all, you can in this session save your reports as default (and primary) setting. Which is the solution of the problem.
Happy to share ...