You can watch this quick video that shows how to clear the web cache. While the icon might look different on different browsers this usually can work. If you can't find the icon and clear cache just let us know and we'll help you further.
https://www.loom.com/share/aa465e86f4694e9eaac52599af938d3e
Once you clear the web cache reload / relaunch the NoteDex app and you should be able to login.
The Windows app leverages web technologies and sometimes we need to clear that. Here is how to do it within the Windows app.