Requisition cancel rejected due to "Exception when processing the change request"

We are on 12.2.5.  A user has a 2-line requisition.  They tried to cancel the entire requisition in iProcurement, but both lines gave a rejection reason of "Exception when processing the change request".  When they went back to query, they saw that line 1 was in Cancelled status, even though it had said the change was rejected.  The cancellation for line 1 rolled up to the PO.

They tried to cancel line 2 and it was rejected again.  They tried both via Cancel Requisition, and Change (select Cancel Line).

There are no receipts or invoices matched to the PO.  Workflow completed normally and did not show any errors.  How can I find more details about why the change is being rejected?