"Password required" popup even though password is correct

Seemingly at random, a popup appears requesting the password for my Yahoo! account, even though I have entered it many times before and it is correct. The explanation is “Server says ‘Invalid username/password’”:

Ps. if I then put the same, correct password in again, the problem goes away for a while, until it next appears.

I get the same issue with my Yahoo account. Not often, but enough to be irritating.

This used to happen all the time with Version 5 and my Yahoo account, but recent releases of Version 6 seem to have almost completely cured it for me. So I am grateful for the progress.

I have seen numerous references by the staff here that Yahoo uses non-standard protocols for connection and verification and can be problematic.

Back when I had the issue, it did seem to help to set the synchronization interval very short, such as every 2 or 3 minutes.

This is absolutely normal issue with Yahoo, they have terrible authentication servers and that is causing these issues.
We have implemented some workarounds but so far we were unable to provide complete solution as it has to be done by Yahoo on their servers.