Kmail - 'read' messages won't stay 'read'

[cr]

Hi, hope this is the right list to ask

I've been using Kmail for some years (under Debian/Gnome :)
I've just installed Deb 4.0r0 on a AMD64 and Kmail's developed a weird bug - 
it keeps marking (some) read messages as 'unread'.  That is, I read 'em (or 
select 'mark as read') and it does the right thing, changes them from blue to 
black - BUT if I swap to another folder, then back to inbox, those messages 
instantly change to blue 'unread' again.    Currently 114 'unread' and 
growing...

I've tried playing with settings, checked permissions & ownership of folders 
in /home/<me>/.kde/share/apps/kmail/mail, anything I can think of, with no 
effect.  (No the partition isn't full  :).

Is this a known bug?   Any suggestions where I should look next?

cr
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