Thanks.
A coworker sent me sample task for uploading content and it looks like the major difference between the task I had written/manipulated and the one they sent is that the session ID is passed to each webject in the task. I created a new task using that methodology and it worked.
The other difference is that the webjects in the task my coworker gave me used the INSTANCE attribute from the start session webject for their INSTANCE attribute data value. I suspect this has nothing to do with the issue becuase it is String value and references the same String variable that I originally was using to set each webject's instance.
Here's a couple of snippets from the new task that worked:
<%
String instance = wt.util.WTProperties.getLocalProperties().getProperty("wt.federation.ie.VMName");
%>
<ie:webject name="Start-Session" type="ACT">
<ie:param name="INSTANCE" data="<%=instance%">"/>
<ie:param name="GROUP_OUT" data="session"/">
<ie:param name="BLOB_COUNT" data="0"/">
</ie:webject>
<ie:webject name="Start-Transaction" type="ACT">
<ie:param name="INSTANCE" data="${session[]INSTANCE[0]}"/">
<ie:param name="SESSION_ID" data="${session[]SESSION_ID[0]}"/">
<ie:param name="BLOB_COUNT" data="0"/">
</ie:webject>
