If you upgraded to Rails 4.1 and you ran the
rake rails:update task, it might have generated an initializer file called
cookies_serializer.rb. By default, it carries the config value
:json, which is incompatible with existing cookies and might cause an error for your visitors.
This has now been corrected and will be released with the next patch release on the 4.1 branch. When upgrading an existing application, the value you want is probably
:hybrid. Consult the upgrade guides for details.