Calender 6 exchange 2010 sp2 and recurring events

I'm experiencing problems with the combination OSX 10.8.2 (fully updated until now), calendar 6 and microsoft exchange 2010 SP2.

First a bit of history to explain what has happened and what I've tried already.

The Mac's in question have been upgraded to 10.8 from 10.6.

At a later stage the mail accounts where migrated from a Kerio mailserver to a hosted exchange 2010 environment.

Mail has been exported from Kerio, imported in exchange.

I removed the old mail accounts on the macs and configured the new exchange account throug System Preferences --> Mail, Contacts & Calendars.

At first everything seemed to work just fine untill the users tried to remove a recurring event. No matter what we tried, the event kept comming back after a while.

I've tried removing and reconfiguring the account.

Removed caches

Remove account, removing caches, rebooting and reconfiguring accounts.

The weird thing is that the appointment is really removed from exchange, when cheking the calendar through outlook webaccess.

Does anyone have a clue how to solve this?