undefined method `transform_keys' loading Sidekiq web

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

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

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