Unable to sign in with my Xbox account

Hi, I'm trying to sign in to Riot Client using my Xbox account, as this is the method I used when I was creating a new account. I signed out as a friend wanted to play the game on my laptop using his account. However, now when I try to sign back in, I'm presented with the page in the attached screenshot. It appears that Riot thinks my Microsoft account isn't an Xbox account, even though this is the account I've been using for about a year. I've tried signing in to the Xbox website as well but it seems that the website is broken as it just sends me back to its home page without signing me in. Because of this I'm now unable to play the game without creating a new account. If anyone can help me regarding this matter, it would be greatly appreciated. Thanks in advance.