It apparently happens that kontact stops responding to any modifications, so i.e. marking a mail as read would no longer result in any reaction.
The application keeps responsive, so you can look at mail etc, but it seems no modifications get executed.
Sometimes akonadi recovers by itself, which results in a storm of modifications getting executed at once, sometimes it doesn't.
From initial debugging output it looks like read-only communication keeps working, but that has to be clarified.