I have a main deco in the living room next to the modem. I have another one in my office connected to the main one via ethernet. The last one is in my bedroom via wifi. 

Initial setup, without any ethernet connections, everything worked in the designated rooms, but the connection was weak, particularly in the bedroom. So I connected the office deco to the main deco with an ethernet cable and that worked. Then, I wanted to change the preferred source of the bedroom to the office (on same floor), but now it keeps flashing a red light. Even after I move it next to either of the deco units. The only thing that works is connecting via ethernet, but I don't have a cable in my bedroom and wifi should be sufficient. 

Now it is stuck with a flashing red light and I can't get it back to work unless I reset it and add it again. 

Making the deco in my office the main deco is not very easy, the hole in the ceiling only fits 1 cable. So that is not the preferred solution. I would expect changing the signal source should just work... 

just now it briefly was green and then went back to blinking red.