I had the same issue for the past week or so. Note that this error only occurred when I would attempt to sign in through the oauth.xfinity.com page. I cleared my browser cache a few times and each try was unsuccessful. I even tried an alternate browser to no avail. However, in my attempt to search for a solution in the forum, I seem to have inadvertently solved my problem.
I was going to chat with a representative, but the chat window required me to sign in. Clicking "Sign in"gave me a "Page not found" error. With chatting not possible, I continued to search and came across a page titled "How Can We Help?" About midway down the page is a "Reset Password" section. Naturally, you need to sign in to change your password. I think it was the sign in link in that section that did the trick. After signing in, I did NOT change my password, instead I went directly to browsing for movies as usual.
Hope this helps some of you.
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