Velero doesn't support self protection, so Velero namespace should not be included into the backup, otherwise, the backup may fail with unpredictable errors.
To help the troubleshooting, and since it will fail anyway, Velero backup should fail earlier when Velero namespace is included.

This should also consider:

• There are multiple instance of Velero installed in the same cluster
• Velero-1 backs up namespace of Velero-2