Redmine

I am a newbie on Rails and Ruby, just work with Redmine for few weeks.
I am configure my redmine to receive email from pop3.
Things are going right but sometime the rake stuck on some particular German email (not all German email).
Please someone help me to solve this problem. I appreciate!

Environment:
Redmine version 2.2.3.stable
Ruby version 1.9.3 (i686-linux)
Rails version 3.2.12
Environment production
Database adapter Mysql2

Below is log output. "isc_CDE1182416-1_70374.pdf" is a attachment.

rake aborted!
undefined method `string' for plain/text; name="isc_CDE1182416-1_70374.pdf":Mail::UnstructuredField
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/field.rb:133:in `method_missing'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/parts_list.rb:48:in `get_order_value'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/parts_list.rb:38:in `block in sort!'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/parts_list.rb:30:in `sort'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/parts_list.rb:30:in `sort'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/parts_list.rb:34:in `sort!'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/message.rb:1690:in `ready_to_send!'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/mail-2.4.4/lib/mail/message.rb:1707:in `encoded'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/actionmailer-3.2.12/lib/action_mailer/base.rb:434:in `set_payload_for_mail'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/actionmailer-3.2.12/lib/action_mailer/base.rb:403:in `block in receive'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/activesupport-3.2.12/lib/active_support/notifications.rb:123:in `block in instrument'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/activesupport-3.2.12/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/activesupport-3.2.12/lib/active_support/notifications.rb:123:in `instrument'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/actionmailer-3.2.12/lib/action_mailer/base.rb:401:in `receive'
/home/linhdinh/redmine-2.2.3-0/apps/redmine/htdocs/app/models/mail_handler.rb:44:in `receive'
/home/linhdinh/redmine-2.2.3-0/apps/redmine/htdocs/lib/redmine/pop3.rb:39:in `block (2 levels) in check'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/1.9.1/net/pop.rb:665:in `each'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/1.9.1/net/pop.rb:665:in `each_mail'
/home/linhdinh/redmine-2.2.3-0/apps/redmine/htdocs/lib/redmine/pop3.rb:36:in `block in check'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/1.9.1/net/pop.rb:533:in `start'
/home/linhdinh/redmine-2.2.3-0/apps/redmine/htdocs/lib/redmine/pop3.rb:31:in `check'
/home/linhdinh/redmine-2.2.3-0/apps/redmine/htdocs/lib/tasks/email.rake:166:in `block (3 levels) in <top (required)>'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:205:in `call'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:205:in `block in execute'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:200:in `each'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:200:in `execute'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:158:in `block in invoke_with_call_chain'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/1.9.1/monitor.rb:211:in `mon_synchronize'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:151:in `invoke_with_call_chain'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/task.rb:144:in `invoke'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:116:in `invoke_task'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `block (2 levels) in top_level'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `each'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:94:in `block in top_level      '
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:88:in `top_level'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:66:in `block in run'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:63:in `run'
/home/linhdinh/redmine-2.2.3-0/ruby/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/bin/rake:33:in `<top (required)>'
/home/linhdinh/redmine-2.2.3-0/ruby/bin/rake:23:in `load'
/home/linhdinh/redmine-2.2.3-0/ruby/bin/rake:23:in `<main>'