Fact that you got a successful response with messages means, your request was fine.

You are missing `apiVersion` field in the [soap request][1]. I believe that field auto-populated by workbench. If you don't specify, sfdc will assume it is first supported version (in my case it was  12). You can check this by viewing the package.xml returned in the retrieve response.


  [1]: https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_retrieve_request.htm