If that’s CIFS, it’s been proving unreliable. A symptom is Duplicati seeing files truncated to even sizes, e.g. 34930688 is a very even 0x2150000. Are you able to confirm the actual size directly on the NAS, and also perhaps from the Pi in some way other than Duplicati’s view? A test of what Duplicati saw after backup is available (with difficulty, perhaps) at <job> → Show log → Remote and finding/clicking the list after file’s initial creation, which you can probably find from its timestamp. Installing sqlitebrowser to look at the local database is probably a quicker way to search for the first. You just put file name at top of the Data column of the RemoteOperation table, and it will find all the rows. It would be interesting to see if the size changes.