@David-TP Thanks a lot DAvid, that solved it. The Deco was in router mode, so the problem was not the ethernet switches but a conflict with my main router (Fritzbox). Here's what I did for others to...

@Networ_kHunter Thanks a lot for your reply, much appreciated. Unfortunately the cabling in my house does not allow me to do this. I can connect the main Deco to the fritzbox, but not the other Decos...

I hae 3 deco units, they are all connected to my main router through LAN. After connecting the office satellite to the main deco, it works for a while, but after several days the light starts...