Hi,
To fix such confirmation, I suggest you read the following KBA note:
1796585 - confirmation status remians in 'Approved' or 'Error in Process'
It contains Z-report to fix different scenarios.
However I am afraid that for confirmation document, since it is transferred using idoc, so the backend error can only be displayed in RZ20 or inbound idoc in ECC side without displayed in SRM confirmation.
You have to add your own enhancement to exchange this backend error to SRM error and raise it after the backend error is returned from ECC BAPI.
BR,
Ivy