пробуйте
mount -v -t cifs //172.20.102.3/backup /mnt/backup -o user=username,password=password,vers=1.0
mount -v -t cifs //172.20.102.3/backup /mnt/backup -o user=username,password=password,vers=2.0
mount -v -t cifs //172.20.102.3/backup /mnt/backup -o user=username,password=password,vers=2.1
mount -v -t cifs //172.20.102.3/backup /mnt/backup -o user=username,password=password,sec=ntlm