Timezone handling is plain horrible, and it seems every CalDAV server
does it slightly differently, also using non-standard headers like
X-WR-TIMEZONE. If you see items being shifted by a few hours, make
really really sure you have properly set org-icalendar-timezone, and
that your calendar is configured to use the same one.
If it still does not work, you can try setting org-icalendar-timezone
to the string "UTC". This will put all events using UTC times and the
server should transpose the time to the timezone you have set in your
calendar preferences. For some servers (like SOGo) this might work
better than setting a "real" timezone.