this.onTimeout is not a function
It turns out this is a known issue with synchronous calls done with AjaxPro's client-side JS. We should probably have ditched it and used regular jQuery AJAX calls on the client side (like here and here) but we haven't gotten around to that yet.
Our workaround was easy - we were making a "fire and forget" call, so it didn't need to be synchronous (we added an empty callback handler to the arg list). We could have also patched the JS ourselves and: