rflickr: invalid auth token when uploading

Hi, I'm working on an application which uses rflickr to upload images to
flickr. I've generated and cached an auth token as described in the
GETTING STARTED document in the rflickr tree. However, when I call the
API, I receive an error stating "ERR: Invalid auth token (98)". The
extended error is:

XMLRPC::FaultException (Invalid auth token):
    /vendor/plugins/rflickr-2006.02.01/lib/flickr/upload.rb:70:in `error'

Any insight into this error will be greatly appreciated.

Here is the action that gets executed on an upload request:

  def upload
    incoming_file = params['flickr_image']
    content_type = incoming_file.content_type.chomp
    msg_out = nil

    responds_to_parent do
      render :update do |page|
        if content_type.rindex(/image\/[(jpe?g)||(gif)||(png)]/)
          cache_file = "#{RAILS_ROOT}/public/cache/flickr.cache"

          if File.exist? cache_file
            flickr = Flickr.new(cache_file)

            msg_out = flickr.photos.upload.upload_image(
              params[:flickr_tags].scan(/"[\w ]+"|\w+/).collect{ |tag|
tag.gsub( /"/, '' ) }
          page.alert "Please upload an image file."

When I inspect the flickr object, I get the following:

"#<Flickr:0xb5c2dfac @person_by_nsid={}, @test=nil, @shared_secret=\"\",
@interestingness=nil, @reflection=nil, @photosets=nil, @api_key=\"\",
@favorites=nil, @auth=nil, @caching=true,
@client=#<XMLRPC::Client:0xb5c2de30 @http_header_extra=nil,
@proxy_port=nil, @parser=nil, @proxy_host=nil, @host=\"www.flickr.com\",
@create=nil, @auth=nil, @user=nil, @port=80, @use_ssl=false,
@cookie=nil, @http=#<Net::HTTP www.flickr.com:80 open=false>,
@password=nil, @timeout=30, @http_last_response=nil,
@endpoint=\"http://www.flickr.com/services/xmlrpc/\", @photos=nil,
@groups=nil, @auth_mode=true, @photoset_by_id={}, @ticket_by_id={},
@contacts=nil, @blog_cache=nil, @group_by_id={}, @urls=nil,
@token_cache=\"/cache/flickr.cache\", @license_cache=nil,
@photopool_by_id={}, @people=nil, @async=false, @photo_by_id={},