I am having the exact same problem. Just sits at 99% complete. In fact I was here just to make that post. I have let it sit for a long time. Also, as you stated, same result if run thru support assist or via web.

edit: I have not tried reinstall. I wanted to see what I could find here first. Glad I didn't try that, doesn't seem to have helped you.

Yes, it's odd isn't it. It's been reliable for Alienware updates up until now. I think it could be related to one of the cumulative or feature updates for Windows 10. All other updates function just fine, Windows, GeForce Experience, appstore etc.

I realise you can manually check on the Dell support site for driver updates etc. - but it was convenient for sure.

I am talking to Dell.

Try also open the My Alienware app - does in open and just a second or two later, does it simply close?

No, I am not seeing that. My Alienware seems to open fine.

Eventually I did get a message thru support assist that an "unexpected error occurred". I did manual checks and I'm reasonably sure I'm at the latest updates, but still I like using the tool to make sure.

Curious what Dell tells you.