Can not turn on "Unlock with Apple Watch" on iPhone

[Mr.Blacky]

Hi. All of a sudden, I can't turn on "Unlock with Apple Watch" on my iPhone. All I get is a spinning circle, but it refuses to turn on.
Any ideas, why it suddenly won't work, even though i didn't do or change anything?

 

[Itinj24]

Have you tried rebooting both devices yet?

 

[Mr.Blacky]

Have you tried rebooting both devices yet?

I did, but no luck.