i have the exact same problem. Fenix 3 has worked fine for 2 years, now suddenly it won't pair. i tried the solution above, as well as hard and soft restarts and pairing with my phone bluetooth system (after restarting phone, forgetting the pairing then re-trying). total failure. i get to the point on the phone where i enter the code. phone think it is connected, watch says try again.
then i disabled the bluetooth for the watch on my phone and tried to pair via the Connect app. swing and a miss. again. right now this thing is dead to me.