I had the same issue, tried everything mentioned in the thread.
Then I went to the Kasa app and saw firmware updates were available for all of the plugs/switches I have.
Installed the updates (two devices failed to install for some reason), restarted HA, and reauthenticated the two devices that I got the notification for.
The error messages went away and the plugs are back in business, although some sensors are unavailable (e.g. overheating).
Update: I’m pretty sure this has to do with the firmware update, the two devices that failed to update the firmware are giving me the authentication error again. Everything else works ok.
I am kind of reliefed. Got crazy not being able to solve the issue but it does not seem to be uniqe. All my TAPO 110’s fail to authenticate giving me unknown error message.
Tapo App → Device → LED state : Change from Auto to Off and back to Auto.
Then unplug the plug and replug in. Then it is available in HA again (needed to reload configuration).
I came in today and was having this problem with my KP125M. NFC automation didnt work and I realized it was asking me to re-auth and I entered my account into but it wouldnt authenticate. I installed the latest HA update and immediately fixed the issue.