Using Classic Outlook on Windows for personal email, calendar, and contact management
You could have been using an office license that was purchased with the pc when you signed into the m365 business basic account, and then when the ups rare happened you signed into your business basic license first, replacing the office license you originally had on your machine, making it so you can no longer use the outlook app any longer with your m365 basic license that only supports outlook for the web. This has happened to me with a few devices that came with office licenses for a year or some sort of package like that.