Hi RB. I'm Greg, an installation specialist, 10 years Windows MVP, and Volunteer Moderator here to help you.
There's a fix for the Gmail Error 404 here:
https://www.youtube.com/watch?v=EzTK68ZdSl8
If you're opening Gmail in a browser, then try another browser to compare. I would have several browsers on your task bar so there's always one that's best for the task. I use Firefox, Edge and Chrome in that order of preference. It's easy to share bookmarks between all of them in their settings under Import Favorites from another Browser, and Firefox and Chrome let you sign in to save and/or sync your Favorites on any other device you sign into. Set the preferred browser as Default in Settings > Apps > Default Apps.
I would also use the Windows 10 Mail app which is an easier way to access your mail from all your various accounts in one huge inbox that lets you merge all of your Office and Web Mail account, with an even larger ad-free reading pane. See here how to get started: https://www.windowscentral.com/how-get-started-...
When adding a Gmail account you need to make a small Security change in Google settings here: https://support.google.com/accounts/answer/6010.... This allows you to use an App that doesn't require you to sign in. To protect yourself be sure to sign in to use Windows which will cover that.
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, I will keep working with you until it's resolved.
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