@need4swede,

It is possible that while the Decos switch between Wireless and Ethernet backhaul a short loop may be created in the network for 1-2 seconds while the transition occurs. This is a behavior that exists as a result of how Mesh technology functions and similar behaviors can be caused by other mesh systems besides your network, such as the mesh used by Sonos Speakers.

If these Deco nodes are wired to your switch, you should be able to disable the Loop Detection feature on the switch ports connected to the Decos. Otherwise, enabling spanning tree after the ethernet backhaul has been established should help.