Can not generate Product Backlog Cards

[eximius313]

platform: 2.3.2 # supported: 2.2.4, 2.3.2
backlogs: 1.0.5 # supported: 1.0.5
ruby: 1.9.3 # supported: 1.9.3, 2.0.0

The browser says:
"Cannot display PDF document"

production.log says:

Started GET "/rb/stories/my_project.pdf" for xx.xx.xxx.xxx at 2013-10-03 16:20:47 +0200
Processing by RbStoriesController#index as PDF
  Parameters: {"project_id"=>"my_project"}
  Current user: my_user (id=3)
  Rendered text template (0.0ms)
Completed 500 Internal Server Error in 10ms (Views: 0.8ms | ActiveRecord: 2.6ms)

Kind regards

[lcguida]

I'm having the same issue. When I click the 'Product Backlog Cards' the log gives me an 500 Internal Error.

My environment:

Environment:
  Redmine version                2.3.3.stable.12228
  Ruby version                   1.9.3-p429 (2013-05-15) [x86_64-linux]
  Rails version                  3.2.13
  Environment                    production
  Database adapter               PostgreSQL
Redmine plugins:
  redmine_backlogs               v1.0.6

Log Message with debug enabled:

Started GET "/rb/stories/optimaster.pdf" for 127.0.0.1 at 2013-10-24 16:48:22 -0200
Processing by RbStoriesController#index as PDF
  Parameters: {"project_id"=>"optimaster"}
  Setting Load (0.5ms)  SELECT "settings".* FROM "settings" WHERE "settings"."name" = 'session_lifetime' LIMIT 1
  Setting Load (0.2ms)  SELECT "settings".* FROM "settings" WHERE "settings"."name" = 'session_timeout' LIMIT 1
   (0.6ms)  SELECT MAX("settings"."updated_on") AS max_id FROM "settings" 
  User Load (41.8ms)  SELECT "users".* FROM "users" WHERE "users"."type" IN ('User', 'AnonymousUser') AND "users"."status" = 1 AND "users"."id" = $1 LIMIT 1  [["id", 1]]
  Current user: admin (id=1)
  Project Load (0.6ms)  SELECT "projects".* FROM "projects" WHERE "projects"."identifier" = 'optimaster' LIMIT 1
  EnabledModule Load (0.5ms)  SELECT name FROM "enabled_modules" WHERE "enabled_modules"."project_id" = 1
  Rendered text template (0.0ms)
Completed 500 Internal Server Error in 214ms (Views: 40.1ms | ActiveRecord: 49.1ms)