Does your FileBackedHTTPMetadataProvider really have no child elements
or did you remove those for posting?
The fact that want to reload metadata that is already configured with
a reloading metadata provider suggests you may be doing it wrong.
Without knowing more, I can't say for sure, but I'll guess that a
LocalDynamicMetadataProvider may be more appropriate in this case. See
the MetadataManagementBestPractices  topic (which is new) for
options and recommendations.