CardDAV Upload Issues - KOLAB16

Hi, I am running Kolab 16 and all runs successfully with Thunderbird, Outlook, Windows Mail, and RoundcubeMail. However, EMClient fails to upload new Contacts correctly or changed existing contacts. No errors in log file or anything and a new contact will appear on the server. The problem is that an undetermined field is missing and as a result the contact cannot be retrieved on any client. I have tried filling in all contact fields in EMClient contacts but the problem persists. Am guessing that it is some identifier string that may be missing or incorrect when creating or uploading to the server. If I edit an existing contact, the changes are not reflected at the server and it will just refresh and bring down the original contact card again. Occassionally it will report Failed To Upload Item, the server returned the following error: Not Found.
Apart from this I am impressed by the overall look and feel of your client software and would like to get this sorted and rolled out to users.
I look forward to your help and support, thank you.