Nuki doorsensor does not connect

I’m trying to connect the door opener.
App does seem to find the opener when battery is inserted, but fails to properly connect.
Phone was restarted, battery was re-insterted, but same result.

Any other ideas or do I simply have a faulty sensor.

Best regards,

I experienced this with two door sensors. In both cases it was a weak battery. (LEDs still flashing but but the voltage seems to low for a connection)
I keep ordering 1/2 AA for my fibaro sensors and some of them always arrive weak or empty.