Steps to Reproduce:

Tried with Microsoft Edge.
I do see GitHub for VSCode as an authorized OAuth app in my GitHub settings.
I'm also getting this if you need an internal repro. Alias is the same as my github account.
cc: @RMacfarlane @shana
@ryzngard I finally got this to authorize after several attempts. What finally seemed to work for me was to revoke the authorization for the GitHub for VSCode app here - https://github.com/settings/applications. Then in VSCode I went to the extensions tab and clicked "reload" on the GitHub Pull Requests extension. That popped up a new "you need to sign in" dialog and that finally succeeded. This was after manually restarting VSCode several times.
Anyhoo, thought I'd pass it along in case it happens to work for you too.
Looks like a new version got pushed that fixed it. I have 0.1.6 now and it worked
This error means that there's a mismatch between the IP that started the login process and the IP that loaded the web page. That could mean that you're running a VPN that's only handling traffic for your browser, or you have some other set up that's causing the IP to change between the two requests.
@shana Thanks, finally got this to authorize after turning off the proxy. I think this reminder should be written on the 810 page.
The only way for me to get past this on enterprise was to follow https://medium.com/@brianhague/viewing-github-pull-requests-in-vscode-with-github-enterprise-859a07e6d353 . I had tried updating the settings with the personal access token already but hadn't tried the very subtle step of closing the authentication popup and just refreshing the PRs inside VSCode which finally did the trick.
Most helpful comment
@shana Thanks, finally got this to authorize after turning off the proxy. I think this reminder should be written on the 810 page.