Why do I now get the error ActionView::MissingTemplate: Missing template user_mailer -- when the template exists already?

Question

On my application I have been successfully sending out email messages, until today. When I run the rake task or run the method mannually from the console, I get the following error:

UserMailer#admin_test: processed outbound mail in 23.6ms
ActionView::MissingTemplate: Missing template user_mailer/admin_test         with "mailer". Searched in:
* "user_mailer"

This would lead me to check if there is a template, which there is in user_mailer/admin_test.html.erb and user_mailer/admin_test.text.erb .

My method in user_mailer.rb is:

def admin_test(user)
  @user = user
  mail(to: @user.email, subject: "Text")
end

Running UserMailer.admin_test(@user).deliver_now! (or just deliver!) renders this error. I initially came upon it running it as a rake task from the scheduler and this error is also replicated straight up from the console.

EDIT

Here's the rest of the error message:

from /Users/xxxx/.rvm/gems/ruby-2.2.1/gems/actionmailer-4.2.1/lib/action_mailer/base.rb:915:in `each_template'

EDIT 2

Okay - so I pushed the current form to heroku and it is working in production but not in development.