The problem with this error is that some record that the system is trying to write to is no longer available in CRM.  For example here is some XML you might see with this error:

<urn2:Message xmlns:urn2="urn:message.data.convio.com" xmlns:urn="urn:soap.sb.convio.com" xmlns:urn1="urn:object.soap.sb.convio.com">
  <urn2:Version>1.0</urn2:Version>
  <urn2:ObjectType>event_attendees</urn2:ObjectType>
  <urn2:Action>UPSERT</urn2:Action>
  <urn2:Origin>
    <urn2:System>Online</urn2:System>
    <urn2:Version>9.2.1</urn2:Version>
  </urn2:Origin>
  <urn2:LocalId>
    <urn2:Id>56781</urn2:Id>
    <urn2:Version>0</urn2:Version>
  </urn2:LocalId>
  <urn2:Data>
    <urn2:Field name="LevelId">a0QF0000006n24HMAQ</urn2:Field>
    <urn2:Field name="GuestCount">1</urn2:Field>
    <urn2:Field name="ConvioSiteId" xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <urn2:Field name="AmountTotal" xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <urn2:Field name="AmountBenefit" xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <urn2:Field name="ConvioTransactionId" xsi:nil="true" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
    <urn2:Field name="ContactLocalId">1032040</urn2:Field>
    <urn2:Field name="ContactUniversalId">003F000000pbzf7IAA</urn2:Field>
    <urn2:Field name="ContactAccountId">001F000000hRY4iIAG</urn2:Field>
  </urn2:Data>
</urn2:Message>


In this case I would check any Saleforce IDs in the above XML to verify that they are all valid.  In the above example I would be checking these lines:

<urn2:Field name="LevelId">a0QF0000006n24HMAQ</urn2:Field>
<urn2:Field name="ContactUniversalId">003F000000pbzf7IAA</urn2:Field>
<urn2:Field name="ContactAccountId">001F000000hRY4iIAG</urn2:Field>

By cutting and pasting the IDs listed into the URL after the ...salesforce.com/ while logged into CRM should bring up each record.  In the above case I found that when I tried going to the name="ContactUniversalId" and the name="ContactAccountId" did not bring up any records and those records where deleted.  

In this case we have a constituent in Online mapped to a constituent record in CRM that no longer exists.  Please contact Blackbaud Support to have the connection to the deleted record removed so a new record in CRM can be created and the Event Attendee information can flow down to the new record.

Note:  The nature of this error is such that a single record with the problem may cause several records to show the same error.  Updates to the Luminate CRM system occur in batches.  For some errors a single reocrd can cause the whole batch to fail and all reocrds would show the same error even though only one record actually has the problem.  Typically the reocrd with the problem is the first one in the list.