When you use a Collaboration Data Objects (CDO) application to update a recurrence task on Microsoft Exchange Server 2010, the update fails. For example, you create a weekly recurrence task and remove a recurrence flag of the task by using a CDO application. After you remove the recurrence flag, you find that the task is still a recurrence task in Microsoft Office Outlook.
When you remove the recurrence flag for a recurrence task, the corresponding recurrence property value of the task is set as "NULL". However, the related recurrence property value is also checked internally. This internal check treats the "NULL" value as an abnormal value.
To resolve this problem, install the following update rollup:
976573 Description of Update Rollup 1 for Exchange Server 2010