But later this week I retried the 712/80 with the current Linux
kernel (4.15.x) and Debian userland and the issue hit me again,
although much later and despite the 100 Mbit network switch in
between. Looking at it I could see that the collision indicator was
active on the switch for the port used by the 712/80. I then
configured a singular port of the 1000 Mbit network switch to 10 Mbit
full duplex and attached the 712/80 to it. And then the issue again
seemed to be gone. But trying to install a package or updating the
package cache again quickly triggered it.
You could try setting the internal NIC to half-duplex, or perhaps use a
(passive) 10BASE-T hub instead of a switch if you cannot configure that
internally, on the kernel command line, or doing it in userland is too
late.