PayPal IPN fails on Meteor app with new host Modulus.io

Question

I have a production Meteor app (v0.8.3). It uses the PayPal IPN system, with a server-side route for the IPN (using Iron Router). I was hosting it with Webfaction and the IPN process worked well.

Router.map(function () {
    this.route('ipn', {
        path: '/ipn',
        where: 'server',
        action: function() {
            if (this.request.method == 'POST') { ... }
        }
    })
})

I recently moved hosting to Modulus.io. I made no other changes. Now the PayPal IPN URL has stopped receiving all calls from Paypal (both in production and in sandbox).

When I do a manual POST via "curl" to the IPN URL, I get a message telling me it has received a POST, so I know the route is working. But I do not get the message when a payment is made via Paypal.

When I use the IPN simulator, it works fine.

And finally, when I deploy the site to a .meteor.com URL, and change the sandbox PayPal IPN to this URL, it also works fine.

Any suggestions on why the live PayPal IPN doesn't seem to be reaching my production IPN URL? Thanks!