Manual upgrade for EBS is not supported and you should follow the steps in (Interoperability Notes EBS 12.0 and 12.1 with Database 11gR2 (Doc ID 1058763.1)) and apply all the patches mentioned in the doc.
What is your OS?
Can you reproduce the issue from different clients and/or using different browsers?
Usually, upgrading the database shouldn't impact the functionality of the forms, however I would suggest you check the application.log and apache log files (error_log and access_log) file for details about the error.
Are there any invalid objects in the database? Any errors in the database log file?