I added a CalDav calendar and at the beginning all the events were displayed on korganizer.
Then korganizer started to miss some of them.
The same missing events are correctly displayed on akonadi console and other applications (e.g. digitalclock calendar widget).
Starting korganizer from CLI does show that all the events are fetched (i.e. "org.kde.akonadi.ETM: was item fetch job: items: xxx"  where xxx counts the total events on the CalDav.
If I delete the calendar and create it again, the same events are missing. The olny way to fix it is to delete all the user configuration files and add the CalDAv again. But then, day by day, korganizer starts to miss some of the events.