When copying large files using SMB, the connection gets suddenly lost and the source computer gets a self-assigned IP address. How is that theoretically possible?

Indeed, it's the Mac Pro.

It's running Mojave (yes, I know about its supported state on the Mac Pro, but I can't think it's the reason). I'll try on El Captain later, but I expect the same issue (that's some behaviours I've seen in the past with all-well supported systems so I bet there's something else).

Thanks. I've checked and AFP is off on both sides (unchecked in the Mac Pro and not even shown in the Mac Mini).

I've the feeling this issue is some kind of “overflow” on the network which confuses “something” and renders the ethernet port not even available to send (or receive?) the DHCP requests. Can this theory be plausible?

Looks like a good catch.

With starting the Mac Pro in safe mode, I was able to copy the remaining files (3 large files) without a problem.

Despite its poor reputation, I have to confess I have CleanMyMac installed on that one. It was part of a bundle I bought, because there were applications with good reputation in it, like Mac Pilot (I thought, at that time, that a bundle wouldn't mix good and bad applications, so I expected all to be good. Won't think this again.)

I've not yet removed CMM because there's still one thing I'm still using that no other software seems to provide (as soon as I find another app for that or the API calls myself, I'll get rid of CMM).

Basically, I have apps that can hang for various “expectable” reasons (my slow and very poor Internet connection may block an app waiting for an answer; or a network share disappearing because another computer was shut down, or a mistake in code of my own apps, etc.). CMM shows when an app/process hangs, in a status menu, and I can investigate right now (to learn the underlying system deeper, also).

The other way would be to keep the activity monitor always open (and visible) to see red entries, but this wastes space and one cannot see all processes at a glance. Granted, hangs are happening not often, but I still like to see and track them down, regardless of its reputation.

I'll now try the same thing with starting normally, quitting CMM and copying again. If it works that way, this would be my first issue with CMM (can fairly well happen…).

Thank you.

Well, it looks like it was it. I've removed CleanMyMac and restarted. Although LittleSnitch told me about a CleanMyMac process which wanted to connect to a server (and already terminated) after I logged in (will have to check, as I've removed all files…), nothing appears in Activity Monitor concerning CMM and I've been able to copy all my files as expected.

And it was not due to a simple restart, as I tried again after safe mode (but prior to removing CMM).

I have to agree about CMM's reputation ☹

Thank you, Grant.