Send email using Actionmailer => error 500 gets rendered :/

I’m experiencing the strangest problem with actionmailer.

The following code reults in a 500 server error.

UserMailer.deliver_register [some arguments]
render :partial => “validate_email” , :status => 200

The strange thing is, Actionmailer sends the email just fine. But
instead of rendering the partial, a 500 error is being returned.
I’ve also tried to render: file => same 500 error. Idem when first
rendering the partial and then sending the email.
Neither the fastcgi, nor the production.log show any errors.

Yet more annoying is that this only happens on my production, using
fcgi, server, using webrick (windows, smtp instead of sendmail) all
works as expected.

Does anyone have the vaguest idea what’s going one?