undefined method `transform_keys' loading Sidekiq web

Update: Rack 2.1.2 has been released and it solves this error

Now the solution is making sure you use 2.1.2 or higher.

bundle update rack

might do the trick for you depending on the rest of your Gemfile.

Two options in order to solve this.

Install a previous working release (recommended). This will install the latest release on 2.0.X branch:

gem 'rack', '~> 2.0.0'

gem 'rack', github: 'rack/rack', ref: 'f690bb71425aa31d7b9b3113829af773950d8ab5'

In both cases, you should remove/replace this with the proper fixed release once this happens. Keep track of https://github.com/rack/rack/pull/1428

it's a rack issue and fixed by this PR: https://github.com/rack/rack/pull/1428

For me, I solved this problem by adding this line to Gemfile

gem 'rack', '2.0.7'

If you have Gemfile.lock in local, you can delete it and then run bundle install.

If you are using Capistrano, you needn't run bundle install manually, and Gemfile.lock often doesn't exists in your repository.

It seems to be an issue with Rack 2.1.1. It works ok with 2.0.8.