Sidekiq web returns a blank page with empty body

Question

I am trying to access the sidekiq web however I am getting a blank page. No error or message.

I am using the following env.:

ruby 2.6.3
Rails 6.0.2.1 (inc. zeitwerk)
rack 2.1.2
sidekiq 6.0.4

If I am grading sidekiq down to version 5.2.7 without zeitwerk, I get no content from the sidekiq web as well. So this must be a generall issue.

If I access the sidekiq web a blank page / html body is returned. Thin show the following requests:

Started GET "/admin/sidekiq" for ::1 at 2020-02-07 11:32:58 +0100
Started GET "/admin/sidekiq/stylesheets/bootstrap.css" for ::1 at 2020-02-07 11:32:58 +0100
Started GET "/admin/sidekiq/stylesheets/application.css" for ::1 at 2020-02-07 11:32:58 +0100
Started GET "/admin/sidekiq/stylesheets/application-dark.css" for ::1 at 2020-02-07 11:32:58 +0100

Any idea is appreciated

Answer 1

rack version 2.1.2 is buggy. I have been downgrading to version 2.0.8.

GemFile

gem 'rack', "2.0.8"

Answer 2

Update

Fixed on new_relic_rpm >= 6.9.0

There was an issue with Rack versions above 2.0.8 where using agent-based Browser instrumentation injection causing Content-Length headers to not match the content size, which in turns causes content truncation in some cases.

https://github.com/newrelic/newrelic-ruby-agent/pull/312