Bundler instruction does not work any more

Even the "fallback" command fails now:

bundle exec jekyll build && (cd _site && python3 -m http.server 3000)

says

Could not find listen-3.9.0, typhoeus-1.4.1, rb-inotify-0.11.1, ffi-1.17.0-x86_64-linux in locally installed gems
Run `bundle install` to install missing gems.

bundle install fails

Bundler 2.4.20 is running, but your lockfile was generated with 2.5.11. Installing Bundler 2.5.11 and restarting using that version.
Fetching gem metadata from https://rubygems.org/.
Fetching bundler 2.5.11
Installing bundler 2.5.11
Fetching gem metadata from https://rubygems.org/.........
Fetching ffi 1.17.0 (x86_64-linux)
Fetching rexml 3.2.8
Fetching faraday 2.9.0

Retrying download gem from https://rubygems.org/ due to error (2/4): Gem::RemoteFetcher::FetchError bad response Forbidden 403 (https://rubygems.org/gems/ffi-1.17.0-x86_64-linux.gem)
Installing faraday 2.9.0
Installing rexml 3.2.8

Retrying download gem from https://rubygems.org/ due to error (3/4): Gem::RemoteFetcher::FetchError bad response Forbidden 403 (https://rubygems.org/gems/ffi-1.17.0-x86_64-linux.gem)

Retrying download gem from https://rubygems.org/ due to error (4/4): Gem::RemoteFetcher::FetchError bad response Forbidden 403 (https://rubygems.org/gems/ffi-1.17.0-x86_64-linux.gem)

Bundler::HTTPError: Could not download gem from https://rubygems.org/ due to underlying error <bad response Forbidden 403 (https://rubygems.org/gems/ffi-1.17.0-x86_64-linux.gem)>
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/rubygems_integration.rb:459:in `rescue in download_gem'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/rubygems_integration.rb:434:in `download_gem'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/source/rubygems.rb:476:in `download_gem'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/source/rubygems.rb:437:in `fetch_gem'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/source/rubygems.rb:421:in `fetch_gem_if_possible'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/source/rubygems.rb:163:in `install'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/installer/gem_installer.rb:54:in `install'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/installer/gem_installer.rb:16:in `install_from_spec'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/installer/parallel_installer.rb:132:in `do_install'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/installer/parallel_installer.rb:123:in `block in worker_pool'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/worker.rb:62:in `apply_func'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/worker.rb:57:in `block in process_queue'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/worker.rb:54:in `loop'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/worker.rb:54:in `process_queue'
  /home/r/Dokumente/Unisachen/iris/iris-project.org/.bundle/ruby/3.1.0/gems/bundler-2.5.11/lib/bundler/worker.rb:90:in `block (2 levels) in create_threads'

An error occurred while installing ffi (1.17.0), and Bundler cannot continue.

In Gemfile:
  github-pages was resolved to 211, which depends on
    github-pages-health-check was resolved to 1.16.1, which depends on
      typhoeus was resolved to 1.4.1, which depends on
        ethon was resolved to 0.16.0, which depends on
          ffi

Can someone who knows bundler figure out instructions that actually work please? :)

Reproduce the documented bundle exec jekyll build and bundle install commands, using the reported Bundler, ffi, and RubyGems errors as the starting point. Locate the project’s Bundler/Jekyll setup instructions, update them so dependencies can be installed and the site can be served, and verify that the documented command works end to end.