I’ve just been trying to upgrade an app to Vaadin 25 and have struck a problem. The app compiles and starts, but after login nothing appears. Looking in the browser console log there is a request for a resource that gets a 404. This is a dynamic resource generated somewhere in the build.
That URL belongs to a StreamResource or DownloadHandler in application code (or an add-on). While the 404 indicates some problem somewhere, it’s unlikely that this would cause the whole page to go blank. The 404 typically happens if the component owning the resource has disappeared, or if there’s some problems with cookies that causes the request to go to the wrong session.
A more likely cause after upgrading is incompatibility in some add-on or custom JavaScript in the application. Do you see any other errors in the browser’s JavaScript console, or errors from the JavaScript bundling process in the server-side log?