After upgraded to Veeam Data Plateform v12 and Upgrading Backup Chain Formats, backup copy jobs failed with :
The target of BCJ are Quantum DXi 4700 with 3.4.x firmware.Failed to preprocess target Error: Unexpected socket shutdown.
I have an open support ticket: 07015562, but no luck, it didn't help me resolve the issue.
I found this https://www.veeam.com/kb4429 and after apply this workaround they are jobs are functionnal.
If can help someone.