Support recently released a new helpful tool to diagnose upgrade issues post upgrade:
The Database Upgrade Troubleshooting Tool.
You’ll access it via MOS Note:559339.1 and click on the top link in the middle column:
The guided workflow has three steps:
1) Describe Problem
2) Upload Files
3) Review Recommendation
Once you have uploaded the upgrade diag log, the upgrade log, alert.log and maybe other things per request of the troubleshooting assistant you should see helpful recommendations.
Try it out if you encounter any issues during database upgrade.