Redmine

I have been experiencing the following 500 error while trying to access some issues.
The problem does not appear on all issues but only on some of them.

My redmine version is 1.2.1 and I also use redmine backlogs plugin.

This is what the production.log shows

Processing IssuesController#show (for XXX.XXX.XXX.XXX at 2012-04-09 16:38:37) [GET]
  Parameters: {"id"=>"699", "action"=>"show", "controller"=>"issues"}
Rendering template within layouts/base
Rendering issues/show.rhtml

ActionView::TemplateError (undefined method `+' for nil:NilClass) on line #45 of app/views/issues/show.rhtml:
42:     <% end %>
43: </tr>
44: <%= render_custom_fields_rows(@issue) %>
45: <%= call_hook(:view_issues_show_details_bottom, :issue => @issue) %>
46: </table>
47:
48: <% if @issue.description? || @issue.attachments.any? -%>

    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:23:in `initialize'
    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:23:in `each'
    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:23:in `initialize'
    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:231:in `new'
    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:231:in `burndown'
    vendor/plugins/redmine_backlogs/app/models/rb_sprint.rb:215:in `activity'
    lib/redmine/hook.rb:63:in `send'
    lib/redmine/hook.rb:63:in `call_hook'
    lib/redmine/hook.rb:63:in `each'
    lib/redmine/hook.rb:63:in `call_hook'
    lib/redmine/hook.rb:60:in `tap'
    lib/redmine/hook.rb:60:in `call_hook'
    lib/redmine/hook.rb:144:in `call_hook'
    app/views/issues/show.rhtml:45:in `_run_rhtml_app47views47issues47show46rhtml'
    app/controllers/issues_controller.rb:122
    app/controllers/issues_controller.rb:121:in `show'
    /usr/lib/ruby/1.8/phusion_passenger/rack/request_handler.rb:95:in `process_request'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_request_handler.rb:207:in `main_loop'
    /usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:374:in `start_request_handler'
    /usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:332:in `handle_spawn_application'
    /usr/lib/ruby/1.8/phusion_passenger/utils.rb:184:in `safe_fork'
    /usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:330:in `handle_spawn_application'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in `__send__'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in `main_loop'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:196:in `start_synchronously'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:163:in `start'
    /usr/lib/ruby/1.8/phusion_passenger/railz/application_spawner.rb:209:in `start'
    /usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:262:in `spawn_rails_application'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:126:in `lookup_or_add'
    /usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:256:in `spawn_rails_application'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:80:in `synchronize'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server_collection.rb:79:in `synchronize'
    /usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:255:in `spawn_rails_application'
    /usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:154:in `spawn_application'
    /usr/lib/ruby/1.8/phusion_passenger/spawn_manager.rb:287:in `handle_spawn_application'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in `__send__'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:352:in `main_loop'
    /usr/lib/ruby/1.8/phusion_passenger/abstract_server.rb:196:in `start_synchronously'
    /usr/lib/phusion_passenger/passenger-spawn-server:61

Rendering /var/www/redmine/public/500.html (500 Internal Server Error)