Contancts are not synchronised

I have a Kerio Connect Server. Emails and calender work well, but after the initial synchronisation the update of contacts does not word any more.

The log view show the following:

09:52:58 agefahrt@smart-em.com [Exchange Web Services]  Synchronisiere Ordner ‘agefahrt@smart-em.com/Contacts/’ 
09:52:58 agefahrt@smart-em.com [Exchange Web Services]  MailClient.Accounts.ConnectionException: Couldn’t get server version.
09:52:58    bei MailClient.Protocols.Exchange.ExchangeAccount.get_ExchangeService()
09:52:58    bei MailClient.Protocols.Exchange.ExchangeItemSynchronizer2.Synchronize(IItemSynchronizeContext1 synchronizeContext, Folder folder, CancellationToken cancellationToken)
09:52:58    bei MailClient.Protocols.Common.ItemSynchronizer`2.<>c__DisplayClass32_0.b__1(WorkerStatus status, CancellationToken cancellationToken)
09:52:58 bei MailClient.Protocols.Exchange.ExchangeGenericCommand.Execute(WorkerStatus status)


There a two new contacts, on the server. But they dose not synchronise down to the eM Client.