The pairing between my Apple Watch and my iPhone broke

@javaliga : thank you for your confirmation of what I thought.

Why the pairing broke ?

I called the Apple suport and they confirmed your point: neither the airplane mode, neither turning Bluetooth

off could explain this pairing break.

Once I was able to repair this pairing break ( see next § ), I could test airplane mode and Bluetooth off to

confirm they are not the right culprits.

Thus the problem is still to be investigated and as it doesn't seem to be yet, the chance it might happen on

many other version of communicating iOS and WatchOS is pretty the same.

I won't be able to help anymore into investigating this bug since I don't know what caused it, and the error

message of the Watch application doesn't give any human usable indication.

Why the pairing isn't fixed from Watch application ?

Concerning the 2nd related problem: impossibility to rebuilt the pairing, the problem was coming from the

fact that my watch was precedently perfectly paired, and the case where this pairing could break was

not foreseen at all within an ideal perfect world ( the one in which software never fail and the last version is

always bug free ).

This case is not documented within the Apple technical note about pairing problems.

Then I had to completly erase my Apple Watch to bring it in a state where it is not paired with anything,

and then restart the pairing process from the Watch application on my iPhone.

This is not a fun process: many times during its progress you might conclude that the process

is dead, there is not the smallest information about what is going on and any estimate of the time to stand

still listening for the time ticking.

All in all it took about 45 minutes.