Failing sync from phone to Synology cardDAV-Server

  • I want to sync my phone (Android 5.0.1) with DAVdroid to an Synology NAS (DSM 6.1.4).
    This was already successful with Synology CalDAV but with the CardDAV-Server from Synology it is not possible to sync from phone to the server. From Server to phone it works well.
    All firewall optios are checked, also PC (Tunderbird, cal & cards) sync with Synology is working without problems.
    As I need two different URLs for the DAVdroid Accounts I need also two different accounts. So it is for me not possible to use the mail account as recommended.
    I assume that is not a problem?
    At first initialising the adresssbook DAVdroid presents two different addressbooks:
    One with the user id as entered for Synology and one other addressbook with /uids/ and a cryptic id. One is automatically named as “<myname> sw” the other as “<myname> sQ”. What does that mean? Both can be synced and appear in the contacts list. Both have the same problem that an upward sync (from phone to server) does not work.
    The error report is as follows:

    <?xml version='1.0' encoding='UTF-8'?>
    <error xmlns='DAV:'>↵
      <no-uid-conflict xmlns='urn:ietf:params:xml:ns:carddav'/>↵
      <error-description xmlns=''>Cannot change the UID in an existing resource.</error-description>↵

    Can somebody help me ? What is going wrong?
    Here is the complete errorinfo

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