Select Extension Element throws file not found

Error:

When performing a data mapping we choose a Salesforce enterprise.wsdl as the target. We right
click on tns:Objects and select Extension Element. In the drop down box we
choose our custom object of Employees__c.

Here's the error from the java console

java.io.FileNotFoundException: https:// localhost:8443/adeptia/mapping?action=getWsExtension&wsType=WsMessageCall&wsId=192168000003138384168906400005&complexType=sObject&baseType={urn:sobject.enterprise.soap.sforce.com}Employees__c&expandLevelCount=5&isBufferingEnabled=true&selfRecursionLevel=0

Cause:

The Tree Expand Level for the extension value is larger than expected

Solution:

Set that value to 3 and then try changing the extension element.