I have been getting thks message frequently with my yahoo accounts... (gmail has never done this to me)
sometimes, I have found that my touchpad has skipped time and the clock is off by 15 min or so. I have to go to "date time" and toggle "network time" off and then back on to reset the time. Then the message will clear.
other times, I have to go to "accounts and prefs" and just click "sign in".
other times the PW is gone and I have to try to remember the last password I used when last fiddled with it.
(and my isp changed their email settings recently too, which caused me additional confusion)
And also I have ATT internet at the house, and they require your smtp settings to go through their servers (though some of my accounts still work if I haven't set it that way), which is another royal PITA.
it all is strange and it seems like its never the same issue. Curious if others have encountered any more issues of this sort, and why this is happening. Another noted there can be multiple reasons for it, but clearly something has been happening relatively recently that causes it, for mysterious, or at least not very clear reasons.