Error on Import using Data Workbench API

[former_member774788]

I'm trying to import an entire business object into the Sales Cloud using the Data Workbench API but I get the following error message:

"Upload error; data file does not match the selected object."

When I import the exact same file using the Browser-UI (i.e. NOT the API), the file is accepted.

The POST body of my HTTP request looks as follows:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<task>
  <projectName>PoC_DataWorkbench</projectName>
  <taskType>1</taskType>
  <operation>2</operation>
  <ignoreBlankValues>true</ignoreBlankValues>
  <simulation>true</simulation>
  <initialLoad>true</initialLoad>
  <serviceName>contract</serviceName>
  <customService>false</customService>
  <rootEntitySetName>ContractCollection</rootEntitySetName>
  <sequence>1</sequence>
  <lastTask>true</lastTask>
  <errorThreshold>30</errorThreshold>
  <dataFile>...ZIP encoded as base64....</dataFile>
</task><br>

I've tried a lot, in various combinations like changing the values of <serviceName>, <rootEntitySetName>, POST'ing without the XML declaration and more but without success.

Does anybody have an idea?