The Data Domain chooses to use a different NIC than configured. Veeam is unable to communicate with the appliance as a result.
This is visible in the job logs:
Info Preparing storage '[REDACTED].vbk' for write. Is optimized for de-duplication media 'False'.
Info [AP] (9150) command: 'prepStorageForWriteEx\nddboost://[REDACTED].vbk\n8192\n0\n'
Info [AP] (9150) error: calling system(), returns nonzero. Err: 5040