herokuでcarrierwaveを使った画像のアップロードだけでなく、文字だけの投稿でもエラーが出てしまう

ローカル環境ではきちんとアップロードできるのですが、herokuにて画像をアップロードしようとすると、エラーが出てしまいます。

※追記
画像アップロードは無しで、文字だけの投稿をしようとしても同様のエラーが出てしまいます。
ログ情報は以下の通りです。

2017-12-13T05:41:37.503141+00:00 app[web.1]: Processing by TweetsController#create as HTML
2017-12-13T05:41:37.503184+00:00 app[web.1]:   Parameters: {"utf8"=>"✓", "authenticity_token"=>"E83/owPz3KwRi34N+89NuykjCi6tmVHPZwM1DcPALx6ZJP6IljBYEUmRVwSR39ep7EqVBlpuFjt+/le7/3KuQg==", "tweet"=>{"text"=>"投稿したテキスト情報。"}, "commit"=>"投稿する"}
2017-12-13T05:41:37.506656+00:00 app[web.1]:   User Load (2.3ms)  SELECT  `users`.* FROM `users` WHERE `users`.`id` = 21  ORDER BY `users`.`id` ASC LIMIT 1
2017-12-13T05:41:37.509520+00:00 app[web.1]:    (1.8ms)  BEGIN
2017-12-13T05:41:37.512904+00:00 app[web.1]:   SQL (2.0ms)  INSERT INTO `tweets` (`text`, `user_id`, `created_at`, `updated_at`) VALUES ('投稿したテキスト情報。', 21, '2017-12-13 05:41:37', '2017-12-13 05:41:37')
2017-12-13T05:41:37.515321+00:00 app[web.1]:    (1.8ms)  COMMIT
2017-12-13T05:41:37.516854+00:00 app[web.1]:   Rendered tweets/create.html.erb within layouts/application (0.5ms)
2017-12-13T05:41:37.518173+00:00 app[web.1]:   Rendered common/_header.html.erb (0.8ms)
2017-12-13T05:41:37.518403+00:00 app[web.1]: Completed 200 OK in 15ms (Views: 2.7ms | ActiveRecord: 7.9ms)
2017-12-13T05:41:37.773991+00:00 heroku[router]: at=info method=GET path="/css/bootstrap.min.css" host=thawing-wildwood-67353.herokuapp.com request_id=38e01828-64b8-40b0-8a4f-935e381448e2 fwd="221.113.38.150" dyno=web.1 connect=1ms service=9ms status=404 bytes=1829 protocol=https
2017-12-13T05:41:42.044501+00:00 heroku[router]: at=info method=GET path="/" host=thawing-wildwood-67353.herokuapp.com request_id=510359d9-352d-4f33-8441-ccf0bd3d22a3 fwd="221.113.38.150" dyno=web.1 connect=1ms service=92ms status=500 bytes=1754 protocol=https

※追記
ローカル環境下では、AWS S3のバケットに画像は保存されています。
herokuでは、エラーが出てしまう状況です。

この現象には、どのような原因が考えられますでしょうか。

index.html.erb

<% @tweets.each do |tweet| %>

  <div class="waku">
  <div class="tweet col-xs-3 col-md-3 block">
    <!-- <div class="row row-eq-height"> -->
    <div class="trim">
      <div class="tweet_image">
        <%= image_tag tweet.image.url %>
      </div>
    </div>
    <div class="tweet_text">
      <%= tweet.text %>
    </div>
    <div class="user">
      <p>投稿者: <%= tweet.user.family_name %> <%= tweet.user.first_name %></p>
    </div>

    <!-- ユーザー自身が投稿した作品の場合に編集・削除・編集ができる-->
    <% if user_signed_in? && current_user.id == tweet.user_id %>
    <div class="dropdown">
      <button class="btn btn-link btn-xs btn-block dropdown-toggle" type="button" data-toggle="dropdown">
        編集・削除
        <span class="caret"></span>
      </button>
      <ul class="dropdown-menu" role="menu">
        <li role="presentation"><%= link_to '編集', "/tweets/#{tweet.id}/edit", method: :get %></li>
        <li role="presentation"><%= link_to '削除', "/tweets/#{tweet.id}", method: :delete %></li>
      </ul>
    </div>
    <% end %>

    <!-- </div> -->
  </div>
  </div>

<% end %>

new.html.erb

<div class="form">
  <div class="col-lg-12">

    <%= form_for(@tweet) do |f| %>
      <h3>
        作品を投稿してみましょう
      </h3>
      <div class="form-group">
        <%= f.text_area :text, placeholder: "作品について入力してください", cols:"30", rows:"10", autofocus:"true", class:"form-control" %>
      </div>
      <div class="form-group file">
        <%= f.file_field :image, class: "form-control floating-label", placeholder: "画像アップロード" %>
      </div>

      <div class="form-group">
        <%= f.submit "投稿する", class:"btn btn-default" %>
      </div>

    <% end %>
  </div>
</div>

image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base

  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  storage :fog
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url(*args)
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   # ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
  #
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end

  # Process files as they are uploaded:
  # process scale: [200, 300]
  process :resize_to_fill => [260, 520]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  version :thumb do
    process resize_to_limit: [260, 260]
  end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  # def extension_whitelist
  #   %w(jpg jpeg gif png)
  # end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

  # 画像の横回転を防ぐ
  # process :fix_rotate
  # def fix_rotate
  #   manipulate! do |img|
  #     img = img.auto_orient
  #     img = yield(img) if block_given?
  #     img
  #   end
  # end

end