OK, something weird happened.
Earlier today I changed the PP acount name for all my yahoo mail.
And I deleted one account.
After that I got out of PP and then restarted PP.
A popup flashed on the screen that said something about password.
PP started, and began to download headers for all my yahoo accounts (4K).
The first time I think it had 77 unread messages in my main account.
The next time it checked mail, it showed 11 unread messages.
The last time it has 0 unread messages in that account.
My main account has 2.5K messages.
Maybe some packets got dropped while downloading 2.5K headers?
Everything looks ok now, no error message

--- Version ---
OS: Windows 7 (x64)
POP Peeper: 4.4.2.0
Webmail: 4.4.4.0
SSL: 1.0.2.11
IMAP: 4.4.2.0
RSS: 4.3.0.0
SendMail: 4.4.2.0
Notification Voice: 4.3.0.0 (disabled)
Profile Picture: 4.3.0.0
AntiJunk: 4.4.0.0
Spell Checker: 4.4.0.0
Web of Trust: 4.4.0.0
--- Error ---
Yahoogordonwilson55IMAP [1000: IMAP] ['imap.mail.yahoo.com' (993) SSL: 1] [Smtp: 'smtp.mail.yahoo.com' (587) SSL: 2] [Consec: 5]
- Error ([UNAVAILABLE] UID FETCH Server error while fetching messages)