Puppet Agents hangs on retrieving plugin

Sometimes the puppet agent hangs on

$ puppet agent --test
info: Retrieving plugin

To solve the problem you can move or delete the process-name.rb file

$ cd /usr/lib/ruby/site_ruby/1.8/puppet/util/instrumentation/listeners/
$ mv process_name.rb process_name.rb.back

Our setup is Ubuntu 12.04 with the common puppet lib. Deleting the file should work.