Template is missingを解決できません

前提・実現したいこと

お問い合わせ機能を実装したいです。

発生している問題・エラーメッセージ

ActionView::MissingTemplate (Missing template layouts/mailer with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :coffee, :jbuilder]}. Searched in:
  * "/home/ubuntu/workspace/app/views"
  * "/usr/local/rvm/gems/ruby-2.3.0/gems/administrate-0.3.0/app/views"
  * "/usr/local/rvm/gems/ruby-2.3.0/gems/kaminari-0.17.0/app/views"
  * "/usr/local/rvm/gems/ruby-2.3.0/gems/devise-4.2.0/app/views"
):

app/mailers/mail_sender_mailer.rb:11:in `inquiry'
app/controllers/contacts_controller.rb:8:in `create'
  Rendering /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/missing_template.html.erb within rescues/layout
  Rendering /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_source.html.erb
  Rendered /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_source.html.erb (5.6ms)
  Rendering /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb
  Rendered /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (2.4ms)
  Rendering /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb
  Rendered /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb (1.6ms)
  Rendered /usr/local/rvm/gems/ruby-2.3.0/gems/actionpack-5.0.0/lib/action_dispatch/middleware/templates/rescues/missing_template.html.erb within rescues/layout (34.2ms)

前も同じ実装をしたのですが、今回は何が原因なのかわかりません。。。

該当のソースコード

【mail_sender_mailer.rb】

class MailSenderMailer < ApplicationMailer

  # Subject can be set in your I18n file at config/locales/en.yml
  # with the following lookup:
  #
  #   en.mail_sender_mailer.inquiry.subject
  #
  def inquiry(contact)
    @contact = contact

    mail to: "メール"
  end
end

【contacts_controller.rb】

class ContactsController < ApplicationController
    def new
        @contact = Contact.new
      end

      def create
        @contact = Contact.new(contact_params)
        MailSenderMailer.inquiry(@contact).deliver 
        respond_to do |format|
             if @contact.save
                        format.html { redirect_to root_path, notice: 'お問い合わせメールが送信されました。' }
             else
               format.html { render action: 'new' }
             end
           end
      end

        private
        def contact_params
          params.require(:contact).permit(:email, :name, :message)
        end
end

【mail_sender_mailer/inquiry.html.erb】

<p>Webサイトからお問い合わせがありました。</p>

--------------------------
Name: <%= @contact.name %><br>
Email: <%= @contact.email %><br>
Message:
<%= @contact.message %>
--------------------------

【mail_sender_mailer/inquiry.text.erb】

Webサイトからお問い合わせがありました。

--------------------------
Name: <%= @contact.name %>
Email: <%= @contact.email %>
Message:
<%= @contact.message %>
--------------------------

【route.rb】

 resource :contacts, only: [:new, :create]

補足情報(言語/FW/ツール等のバージョンなど)

こちらを参考に作成しました。

追記

【application.rb】

# config.i18n.default_locale = :de

【locale】

devise.en.yml
en.yml

【エラー内容】

Missing template layouts/mailer with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :coffee, :jbuilder]}. Searched in: * "/home/ubuntu/workspace/app/views" * "/usr/local/rvm/gems/ruby-2.3.0/gems/administrate-0.3.0/app/views" * "/usr/local/rvm/gems/ruby-2.3.0/gems/kaminari-0.17.0/app/views" * "/usr/local/rvm/gems/ruby-2.3.0/gems/devise-4.2.0/app/views"