503 due to content-length error? microwave issue?

I have nicely functioning varnish instance in front of a drupal/civicrm website, running on debian/lenny (varnish-2.1.3 SVN ). I have one user who is unable to POST (though maybe by AJAX . not sure ...?). When she tries, she gets the varnish 503, which doesn't help too much. When I look at the

Is Varnish useless with slow/remote back-ends large files?

Hi! I am attempting to use Varnish in a somewhat unusual fashion, and before I give up and drop it for something else, I figured I'd describe my problem here and see if there's something I've missed. What I am doing is implementing a service (http://pagekite.net/) to provide a remote HTTP