Hi, I'm having trouble creating or upserting a junction object via the API, the object is custom.
The problem seems to be restricted to junction objects (when it has 2 Master-detail relationships in it), because any other type of custom object works perfectly fine.

Here's a successful XML request :

<soap:Body>
<sforce:upsert xmlns:sforce="urn:partner.soap.sforce.com" xmlns:sfons="urn:sobject.partner.soap.sforce.com">
  <sforce:externalIDFieldName xsi:type="xsd:string">ID</sforce:externalIDFieldName>
  <sObjects xsi:type="sforce:AcctAttnAssoc__c">
   <sforce:AttendanceDetails__c xsi:type="xsd:string">a194000000040deAAA</sforce:AttendanceDetails__c>
   <sforce:Name xsi:type="xsd:string">randomname</sforce:Name>
   <sforce:Account__c xsi:type="xsd:string">0014000000rE6moAAC</sforce:Account__c>
  </sObjects>
</sforce:upsert>
</soap:Body>

And here's an unsuccessful XML request:

<soap:Body>
<sforce:upsert xmlns:sforce="urn:partner.soap.sforce.com" xmlns:sfons="urn:sobject.partner.soap.sforce.com">
  <sforce:externalIDFieldName xsi:type="xsd:string">ID</sforce:externalIDFieldName>
  <sObjects xsi:type="sforce:Attendance_Log__c">
   <sforce:Attendance__c xsi:type="xsd:string">a1340000008DlmxAAC</sforce:Attendance__c>
   <sforce:Name xsi:type="xsd:string">randomname</sforce:Name>
   <sforce:Account__c xsi:type="xsd:string">0014000000rE6moAAC</sforce:Account__c>
  </sObjects>
</sforce:upsert>
</soap:Body>

As mentioned before, the only difference is that AcctAttnAssoc__c has 2 Master-detail relationships in it (Attendance__c & Account__c).
The error I get back is:

Fatal error: INVALID_TYPE: sObject type 'AcctAttnAssoc__c' is not supported. If you are attempting to use a custom object, be sure to append the '__c' after the entity name. Please reference your WSDL or the describe call for the appropriate names.

Please let me know if you need any more details, thanks.