@Renard 

I see on the user manual flashing blue means ready for setup and I think this E4 might auto reset itself occasionally.

I think you could try to set it up as the main Deco to see if the same issue continues.

If yes, you might have a broken unit.

  @Heidi_Alp 

I have tried switching it to become the main deco and the same issue is still present, constant blue flashing light. 

It also appears to have set up its own network

  @Renard 

Hi, I see there is also an Open SSID-Deco-1994 and have you checked the MAC address of this flahing blue unit to see whether the last 4 MAC is also 1994?

And I noticed you have reseted it before, did it response when you reset it, like a LED change or it still stayed flashing blue no matter how?