HTC TyTN II/Kaiser: Calendar sync problems

[McYoda]

Hi there

I just started with a new employer where we use zimbra collaboration suite. Privately, I use MS Outlook, which I sync with my Pocket PC (WM6) which is a HTC TyTN II a.k.a. Kaiser.
I successfully configured ActiveSync to connect to our zimbra server. All my contacts got synched from HTC to zimbra, but no calendar appointments got synched.
To test, I created a new appointment in zimbra. This will be synched back to my phone. When I create an appointment on the phone it will neither be synched with zimbra.
What could be the reason, my phone appointments do not get over to zimbra but my zimbra appointments will arrive on my phone?

Thanks for any hints
McYoda