This is the patch used to get around the bug reported with this gist: https://gist.github.com/fractaledmind/7574eaeb00dff6b9afd7778a7e024c19

paper_trail_bug_patch.rb

# We need to patch PaperTrail to deal with Campaign having a `serialize` field that also has an `attribute` definition with a `default` value set for the attribute, as well as a database-level `default` set as well. For whatever all the reasons, PaperTrail tries to serialize the database-default, which is already a string. This throws an `ActiveRecord::SerializationTypeMismatch` error, as it is expecting the valued being serialized to be an `Array`. In this case, we can solve our problem by simply catching this error and checking of the value is already serialized.
module PaperTrail
  module AttributeSerializers
    class CastAttributeSerializer
      private

      def serialize(attr, val)
        AttributeSerializerFactory.for(@klass, attr).serialize(val)
      rescue ActiveRecord::SerializationTypeMismatch => error
        if val.is_a?(String) && YAML.load(val)
          val
        else
          raise error
        end
      end
    end
  end
end