Overview: 

Using Korganizer to setup a new CalDav resource it seems to:
- Collections are read from the server
- Existing calendar events are not retrieved from the server
- Locally created calendar events :
    - are written to the server 
    - local changes are written to the server
    - changes to same events, including delete, on other devices ( Kubuntu 17.04, Android phones an tablets, mozilla Thunderbird) are not read from sever
    
- Akonadi console shows only number of locally create items. In all other cases it shows 0 retrieved items from server.
  Reading the debugger output within the Akonadi Console I don't see any error messages except the "Information view" shows:
  "AgentBase(akonadi_davgroupware_resource_2): Unable to retrieve items: There was a problem with the request. HTTP error (0)."


Version info:
OS : KDE Neon 5.11
KDE plasma version : 5.11.3
KDE Frameworks : 5.40.0
QT version : 5.9.1
Korganizer 5.6.3
I don't have detailed information about the caldav back-end. My provider only tells me that it is an Open-Xchange server.




Additional Information: 
As already mentioned above using 
Kubuntu 17.04
KDE plasma version : 5.10.5
KDE Frameworks : 5.38.0
QT version : 5.7.1
Korganizer 5.4.3
and other devices everything works fine.

Extra info
KDE Neon 5.11 : new installation
Kubuntu 17.04 : upgraded from previous versions

When upgrading 17.04 to 17.10 the caldav resource is also broken.