This is because the app from upgraded from Mendix 8 to Mendix 9. Creating a new app in Mendix 9 works. The problem you are seeing is down to a missing tag in the index.html file that was added in Mendix 9. In the <head> add the following…
{{unsupportedbrowser}}
This will add JavaScript that looks for modern code and if not supported it will redirect the user to the unsupported-browser.html page.