@KingKoen 

There must be networking switch connected to the second port of Main Deco, and Satellite Decos as well as other hardwired devices on your network pass Internet traffic through that switch to the Main Deco.

If this is the case, the following scenario could explain what you see: that switch lost Ethernet connection to Main Deco.

For example, Ethernet cable connecting switch to Main Deco got unplugged accidentally or damaged. When that happens, Deco mesh will elect one of Satellites communicate over WiFi to Main Deco, and all your Internet traffic will actually pass through that switch to that Satellite and wirelessly to Main Deco.

If you power off that Satellite Deco, another Satellite Deco will switch to WiFi backhaul. 

This scenario is quite easy to test if you have hardwired devices other than Satellite Decos. Power off all Satellite Decos and these hardwired devices should lose Internet connection, because switch does not have Ethernet link to Main Deco.

  @Alexandre. 

This looks like what is happening. The whole setup looks like this:

Router

     +---- Main Deco [1]

     +---- Deco [2]

     +---- Deco [3]

     +---- Switch

                 +---- Deco [4]

                 +---- Switch

                             +---- Deco [5]

Do the other Deco's (2-5) need to be connected to the second ethernet port of the main Deco (1) instead of via the router?

All Deco's are wired.

  @KingKoen 

You have different network setup than I expected. This is what I expected:

The scenario I described is applicable to this setup, and not to what you have. Sorry, out of ideas with what would cause your issue.

  @Alexandre. 

I unfortunatly cannot make it this way as I have a combine modem/router/switch device...

Thx for your $0.05 ;-)

  @Alexandre. 

In the end your comment was valid. I connected a switch to the second port of the main deco and connected all others (in)directly to that switch. Now all use ethernet as backhaul.