Missing calendar items from CalDAV ressource

I'm using DavMail [1] as a middleware between Akonadi and my company's Microsoft Exchange server. In order to get my calendar shown in Kontact, I have set up a CalDAV resource, pointing to DavMail's local URL. Most of my VEvents show up fine, except for some events (both repeating and non-repeating events).

When I download the calendar manually (by opening the calendar URL in a browser) and then add it as iCal ressource, all events are shown, including those who are missing in the CalDAV ressource.

I tried to find out how to debug this, to see if something is going wrong during import, but I do not know how to do this properly:

- I ran "akonadictl restart" from the command line, to get some debugging output
- I ran "gdb akonadi_control" and added the CalDAV ressource
- I followed the "Debugging Akonadi Agents" tutorial [2]

Maybe I'm looking at the wrong place?

[1] http://davmail.sourceforge.net/
[2] https://techbase.kde.org/Projects/PIM/Akonadi/Development_Tools#Attaching_gdb_at_startup

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: akonadi-server 1.13.0-2ubuntu4
ProcVersionSignature: Ubuntu 3.19.0-47.53-generic 3.19.8-ckt10
Uname: Linux 3.19.0-47-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.8
Architecture: amd64
CurrentDesktop: KDE
Date: Mon Jan 25 10:10:55 2016
InstallationDate: Installed on 2015-10-13 (104 days ago)
InstallationMedia: Kubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
JournalErrors: Error: command ['journalctl', '-b', '--priority', 'warning'] failed with exit code 1: No journal files were found.
SourcePackage: akonadi
UpgradeStatus: No upgrade log present (probably fresh install)