"message authentication code incorrect" error with scp

This looks like communication network issue to me. "MAC incorrect" probably means a packet gets damaged. That could be caused by several factors.

I had similar problem with BareOS, backups simply failed. In that case I disabled some hardware offload features on one machine, and failures vanished:

(in the /etc/network/interfaces)

iface vmbr0 inet static
        ...
        bridge-ports eno1
        up ethtool -K eno1 tso off gro off gso off tx off rx off ufo off

Another causes may be faulty WiFi. Try another wlan card, another wireless channel. Other one is broken wired switch.

The general solution is to stress test your network and fix any found issues.