The error message: ‘Unsupported Message “This app can’t be loaded”’ can occur when your app is configured on Shopify’s backend to be in “legacy mode” (you can easily detect an app is in legacy mode if the title bar does not show the icon of the app and displays menu buttons in a single bar instead of on two bars).
@Alex I’m having a similar issue with my app ID 1381729. It’s an older embedded app that I’m trying to move to App Bridge from EASDK. There are no references ShopifyApp and all the old EASDK app code has been removed (also not loading the javascript).
Not sure if I’m doing something else wrong here to not get out of legacy mode but a forced switch would be very helpful.