I have seven Kasa plugs of various types - I have listed three examples - on two different WiFi networks in our house and in the garden office. Last week ALL of them stopped working with Alexa after having been fine for several years. In the Alexa app, the Kasa skill says "The link between accounts has expired" and "Enable to use". When I click Enable, it gives some blurb and has an Agree & Link button. When I press this I get a pop-up saying "Network is unstable". It always says this, repeatedly, hour after hour and day after day. The network is not unstable, everything else (dozens of devices on the two networks) are working normally. 

All the plugs work in the Kasa app as usual, but that's no help to everyone else in the house and the various Alexa groups they're in.