Netconf returns bad-element: config

Question

The configuration:

<config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">  
   <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
      <interface>
        <name>Loopback138</name>
        <description>TEST_SPSJM</description>
        <type xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type">ianaift:softwareLoopback</type>
        <enabled>true</enabled>
      </interface>
  </interfaces>
</config>

the ncclient cmd:

nc_manager.edit_config(conf, default_operation="merge", target="running")

the response:

<bad-element>config</bad-element>

this however definitely worked some time ago.