I think the example in the README for Rally-Lookback-Toolkit is misleading/erroneous in the way it specifies a Workspace by name:
lookbackApi.setWorkspace("myworkspace");
When I do this, I see the following error from LBAPI:
Lookback Exception com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1
What's needed in the REST URL is actually the Workspace ObjectID
instead of the name. Thus, if you set the Workspace ObjectID in your code:
this.lookback.setWorkspace("12345678910");
I think the query should work for you.