Hi,
You can make the 'error message' to 'warning message' through Tcode OBA5 and proceed with the transaction. We have done the same and it is working fine.
To analyse the error, while maintaining the settlement rule in AIAB, click on Group List(F5), there you will see that you have settled to the same asset earlier i.e. sender and receiver is the same and now after upgrade it is showing as error.
It is mostly probably an upgrade issue.
Regards,
Ramnik