I have never finished a backup.
I use CIFS folder, used before with scp file transfer from the same server without any problem. I attach here my backup HDD benchmark (I Know, it is not faster than other, but it is for backup).

Each time I run the backup I have different bottlecages: one time proxy, one time destination, one time source..... so for me it is impossible to understand.
Here I attach also veeam logs, syslog and kernel dump. link
Thank you for your investigation
Andrea