After years without a problem, I now can't Sign In to my account & connect to Live from my Xbox 360 console.
It asks for "proof" and requests I enter my email & password. Once that is done, while sitting on the "Save Password" screen it does appear to sign in as I get all the "You have X friends online" notifications. It then errors out when you try to click Next on the "Save Password?" screen.
I know the email and password are correct as they log me in to the website. Also because the exact same issue is happening for all Live Accounts on my console.
"Test Connection" from the Settings says it is connecting fine at all stages of the process.
I changed my Xbox Live password. Didn't help.
I changed to 2 Step Verification and entered the App Code it gave me instead of the password. It didn't help.
I'm at a bit of a loss as to what to do next. It's effectively bricked my console as I can't get at any of my save games even to play offline.
This has coincided with downloading the GTAV Online update though this could easily be a coincidence.