Delayed_job - Undefined method error even with Mongrel

Any delayed_job gurus out there? This is a known issue on the wiki as
described at the bottom here:

However, even when running Mongrel (on Rails 3.0.3, ruby 1.9.2, Win7)
after replacing Webrick) I am still getting this issue. My delayed_job
handlers read:

— !ruby/struct:Delayed::PerformableMailer
object: !ruby/object:Class UserMailer

when it should read:

— !ruby/struct:Delayed::PerformableMailer
object: !ruby/class UserMailer

Is there a solution to this problem other than running mongrel?
Because I am running mongrel and still have the same problem. Thanks in