If you have your code defined in classes in lib/ folder you may have problems to load that code in production.
Autoloading is disabled in the production environment by default because of thread safety.
Change config/application.rb:
config.autoload_paths << Rails.root.join("lib")
config.eager_load_paths << Rails.root.join("lib")
It works in development and production environments. eager_load_paths will get eagerly loaded in production and on-demand in development. Doing it this way, you don't need to require every file explicitly.
Recommendations: (from https://stackoverflow.com/questions/38198668/rails-5-load-lib-files-in-production)
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Place lib dir into app because all code inside app is autoloaded in dev and eager loaded in prod and most importantly is autoreloaded in development so you don't have to restart server each time you make changes.
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Remove any require statements pointing to your own classes inside lib because they all are autoloaded anyway if their file/dir naming are correct, and if you leave require statements it can break autoreloading. More info here
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Set config.eager_load = true in all environments to see code loading problems eagerly in dev.
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Use Rails.application.eager_load! before playing with threads to avoid "circular dependency" errors.
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If you have any ruby/rails extensions then leave that code inside old lib directory and load them manually from initializer. This will ensure that extensions are loaded before your further logic that can depend on it:
# config/initializers/extensions.rb
Dir["#{Rails.root}/lib/ruby_ext/*.rb"].each { |file| require file }
Dir["#{Rails.root}/lib/rails_ext/*.rb"].each { |file| require file }
References: