Failed to use mongrel on Windows server with Rails 3 and Ruby 2.1

I'm trying to install redmine on a windows 2008 server. It works with webrick. Now I'm trying to put it in production with mongrel. But when I start mongrel I have the following error:

E:/Ruby/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in require': cannot load such file -- 2.1/http11 (LoadError) from E:/Ruby/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:inrequire' from E:/Ruby/Ruby21/lib/ruby/gems/2.1.0/gems/mongrel-1.2.0.pre2-x86-mingw32/lib/mongrel.rb:17:in rescue in \<top (required)>' from E:/Ruby/Ruby21/lib/ruby/gems/2.1.0/gems/mongrel-1.2.0.pre2-x86-mingw32/lib/mongrel.rb:13:in\' from E:/Ruby/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in require' from E:/Ruby/Ruby21/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:inrequire' from E:/Ruby/Ruby21/lib/ruby/gems/2.1.0/gems/mongrel-1.2.0.pre2-x86-mingw32/bin/mongrel_rails:13:in \<top (required)>' from E:/Ruby/Ruby21/bin/mongrel_rails:23:inload' from E:/Ruby/Ruby21/bin/mongrel_rails:23:in `'

I don't really know Ruby and can't figure out what the error means.

Thanks for your help

I'm not familiar with redmine, but I think problem is that you want to run Mongrel server, which is very old, and maybe not compatible with ruby2 (which you use). You can try add to Gemfile.local gem 'thin', for example, and make bundle install. I hope this manual will help you.