@welikegoats 

I would suggest to replace Ethernet cable. If it works now, just leave it as is, but if/when same issue comes back - the first step should be Ethernet cable replacement.

  @Alexandre. I would have thought the level of detail I provided of the troubleshooting I went through would suggest that I may have done this sort of thing once or twice before. There is no problem with the cable, it occurred as a direct consequence of the update and it was resolved when I used a different Ethernet port.

  @welikegoats 

You do realize that "using different port" means you had to unplug/plug back cable and perhaps move cable a little, bend it a bit, etc. If cable is borderline to fail, especially around the connector, just doing that could have suddenly helped and different port has nothing to do with it.

Prove me wrong with another test:

1) Connect that cable to Decos how it were originally.

2) If nothing works again, connect cable the way you have now. If it started to work again, then I'll admit my mistake.

  @Alexandre. Done. The effects I described are consistent.

  @welikegoats 

As promised, I admit my mistake.

Recently, I had an issue with Main Deco dropping speed from gigabit to 100Mbps. It appears for some reason Ethernet cable was not plugged tightly to Main Deco or got loose, just unplugging it and plugging back to the same Ethernet port restored gigabit speed.

  @Alexandre. I'm trying to raise this as a bug report. Have I succeeded?

  @welikegoats 

I've noticed that TP-Link Support usually does not check that forum on weekends. 

  @David-TP I'm not really in a position to swap the location of the two X20s, but I do have some more information. The second X20 was connected on Ethernet port 1 to the first X20. If I move the cable to Ethernet port 2 on the second X20, I get much less throughput (down to 100Mb/s) but no packet loss. If I move it back to port 1 I get ~400Mb/s.