Sync CalDav error: Index was outside the bounds of the array

Getting error on syncing one of many calendar account with message:
Index was outside the bounds of the array.

Full error log shows:
[CalDAV / CardDAV]Während der Synchronisation des CalDAV-Ordners ist ein Fehler aufgetreten:System.IndexOutOfRangeException: Index was outside the bounds of the array.
at System.String.get_Chars(Int32 index)
at MailClient.VObject.CalendarReader.StringToDuration(String value)
at MailClient.VObject.CalendarReader.LoadDurationValues(String encodedValue)
at MailClient.VObject.CalendarReader.LoadValues(ValueType type, Encoding encoding, Nullable1 separator) at MailClient.VObject.CalendarReader.LoadProperty(String line) at MailClient.VObject.CalendarReader.LoadFromReader(Calendar calendar) at MailClient.VObject.CalendarReader.LoadCalendar(Calendar calendar) at MailClient.VObject.Calendar..ctor(TextReader reader) at MailClient.Protocols.CalDav.Synchronizer.SynchronizeItemsCommand.CreateCalDavItem(Stream payloadStream, Encoding encoding, Uri uri, String eTag) at MailClient.Protocols.CalDav.Synchronizer.SynchronizeItemsCommand.CreateCalDavItem(ResponseItem item, Uri responseUri) at MailClient.Protocols.CalDav.Synchronizer.SynchronizeItemsCommand.DownloadItemsMultiget(IList1 uris, Boolean calDav)
at MailClient.Protocols.CalDav.Synchronizer.SynchronizeItemsCommand.ProcessItems(Dictionary`2 items, Boolean deltaSyncReport)
at MailClient.Protocols.CalDav.Synchronizer.SynchronizeItemsCommand.Execute(WorkerStatus status)

Any idea how this could be resolved? - all other Calendar accounts syncing just fine.
Account deletion and reconfigure or cleaning local cache did not solve the issue.

Regards
Jörg