Apple Watch suddenly says charging not supported with this accessory.

Are the watches up to date?

On the iPhone tap the Watch icon then tap General > Software Update

Latest OS is 2.2.2.

I bought my Apple Watch last June. It was working just fine until immediately after updating to WatchOs3. First thing it told me after the update had finished: "charging not supported with this accessory". (Mind you: I am using the charger that came with with the Watch!). Apparently my watch refuses to digest the update to Os3 because not only did it tell me the charger is not supported it seems tot have a life of its own when it comes to charging and power. There are time it does not charge at all but the watch becomes very warm (almost hot); sometimes it drains to zero from a full charge in under two hours. Sometimes it switches to the power reserve mode all by itself. And sometimes the display stays on when on the charger end even when it is not on the charger causing it to (again) drain itself in a matter of hours.

I went to the Amsterdam Apple Store this evening where they ran a diagnostics program. Outcome: there's nothing we can find! Now they've sent it to be examined elsewhere. Real bummer.

My watch just did this also. The software is current and up to date, and I'm using the Apple Watch charger included in the box with it. It seems to be charging now, but it took quite a few times taking it off the charger and putting it back on for it to register and begin charging or see it as an authorized charger.