In my dashboard, I see:
There are 1409 email jobs that failed. Check your app.yml and ensure that the mail server settings are correct. See the failed jobs in Sidekiq.
When I click through to the failed jobs, I see
Jobs::HandledExceptionWrapper: Wrapped NoMethodError: undefined method `with_secure_media?' for #<Post:0x00007f48bd49d9c0>
The problem seems to correlate with an upgrade to v2.4.0.beta8.