Outlook Integration: cannot insert the value NULL into column

In a NAV2009 DB I added a custom field ‘fCodComplaintNo’ to T5080 To-do. Field type is Code.
When a user synchronizes Outlook with NAV, the following error is generated:

Microsoft Dynamics NAV Codeunit Execution for response Error: the following SQL server error or errors occured when accessing the To-do table:
515,“23000”,[Microsoft][ODBC SQL Server Driver][SQL Server]Cannot insert the value NULL into column fCodComplaintNo.

There are a lot of code fields in this table that do not cause this error to be generated. What could be the cause of this? Or how should I avoid this field being updated in the Outlook Sync process?

Solved by restarting the NAV Service.