@Henkan 

Well, you gave network layout that does not mention second switch. You ask what could be the problem when connecting Satellite Deco to that second switch.

You need to provide more details. To start with: what is that switch brand and model name, where it is connected?

Does the below make more sense? 

Deco Office (main, in Router mode):
1 Internet (fiber, 500 Gb)
2 Switch Office

Switch Office (Zyxel GS-108B v3 8-ports):
1 (Not used)
2 Ethernet Surface Dock
3 Switch Floor 1 port 3
4 Switch FLoor 1 port 4
5 TV
6 Mac Mini TV
7 Switch Basement
8 Deco Office (Main)

Switch Basement (Zyxel GS-108B v3 8-ports):
1 Plejd Gateway
2 (Not used)
3 (Not used)
4 (Not used)
5 (Not used)
6 Heat pump
7 Mac Pro in Studio
8 Switch Office

Switch Floor 1 (Zyxel GS1100-24EV3 24-port Gigabit Switch):
1
2
3 Switch Office port 3
4 Switch Office port 4
5
6
7
8
9 Ethernet port 1 Bedroom 3 floor 1
10 Ethernet port 2 Bedroom 3 floor 1

  @Henkan 

This helps, thanks.

As far as Switch Office concerned, you have both Main Deco and Satelltie Deco (from the basement) connected to it. This is not going to work. See TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco

To fix that, you either need to configure Deco mesh to run in Access Point mode (but it will require router in front of Main Deco) or rewire your network, for example this way:

Internet (fiber, 500 Gb) <--->Deco Office (Main)<---> Switch Office

  @Alexandre. I *think* we are talking about the same thing, but I drew a fast laout of the network just to be sure. The Basement Deco works (somewhat ok) but when I connect the 1st Floor Deco everything goes south. When I use it wirelessly it works though.

On thought I got is that it could be that the basement switch is connected with two cables from the office switch? Is this a possible source of error?

  @Henkan 

Let's start with troubleshooting, then.

1. Check in Deco app if Satellite Deco Basement reports Ethernet backhaul, you should see it in Satellite Deco info page:

2. If it does, take Satellite Deco Floor 1 to the basement, wire it to Switch Basement:

A) Do you have stable Deco mesh?

B) Does Satellite Deco Floor 1 report Ethernet backhaul now?

  @Alexandre. I disconnected the second Ethernet connection from the office switch to the first floor switch (I have no idea why the installer installed it?) and after that the first floor Deco connects via Ethernet without any problems (as do the basement Deco). It seems like the second connection resulted in networks loops. For now, everything works as I intended. Thanks for all help!