I've been going through a weird problem lately. I have two boxes connected to a D-Link home switch.

Here's an outline of what they are and how they're connected:

B1:

- Distributor ID: LinuxMint
- Description: Linux Mint 19.1 Tessa
- Release: 19.1
- Codename: tessa
- 192.168.25.70 in a /24 network (ordinary home network);
- IP assignment defined by router (DHCP)

It is normally connected with cable and works as a desktop form where I do all my stuff (pay bills, research stuff, etc...)

And...

B2:

- Distributor ID: Ubuntu
- Description: Ubuntu 18.04.4 LTS
- Release: 18.04
- Codename: bionic
- 192.168.25.249 in the same /24 network
- IP defined in local config

The thing is:

When I connect a network cable to machine B1, the services on machine B2 become unconnectable (SSH, Apache, etc). However, as weird as this may seem, I can still send and receive ICMP echo requests and receive echo replies to/from B2.

I thought maybe some weird DHCP service could be running inside B1 or B2, but no...

The plot thickens as the problem only occurs when both of them are connected through cable. For instance, when I connect B1 to the wireless network and leave the cable connected to B2, everything works fine, as I can access all the services (SSH, Apache, etc.).

I tried modifying the cable connections on the switch hoping it would be something related to the ports and nothing worked.

Do you have another switch test with? Seems like that might be the problem, assuming your config for B2 is correct.

It seems to be, because I am being able to connect to it now (since B1 is connected to the Wifi).
I have another switch here and will give it a try.