Adobe Assets extension won't connect - Unhandled Rejection (Error): Timed out waiting for initial response from parent

I'm able to open up my extension in Adobe Experience Manage Assets, which is running on my local server. I can briefly make changes locally which are reflected in the extension, but after 20 seconds it times out with:

When I open up my local application at https://localhost:9080,  I get "IFrame for integration with Host (AEM Assets View)...", then it times out again with the same error. I've tried creating the bootstrapped app multiple times using this guide: https://developer.adobe.com/uix/docs/services/aem-assets-view/extension-development/, but no luck. SSL certificate has been accepted.