On screen: There was a problem and we couldn't continue (0x80A40008)
This one genuinely goes both ways, so the first job is telling which you are looking at.
Microsoft gives four causes: an Xbox Live connectivity issue, wrong Microsoft account details, a problem with your Microsoft account itself, or too many accounts saved to the console.
That last one is the interesting one. A console that has accumulated sign-ins over years can hit a limit, and no amount of network work addresses it.
Microsoft gives you the test themselves. Go to Settings, General, Network settings and run Test network connection. If it reports everything is good, the fault is the account rather than the network. Then try signing in at account.microsoft.com in a browser: if that works, the account is healthy and you are looking at cause two or four.
Microsoft's final step for this code is a full factory reset, and their own warning is worth quoting exactly: restoring your console to the factory default settings erases all accounts, apps, games, saved games, and settings, and anything not synchronized with Xbox Live will be lost. This is the destructive option without the keep-my-games choice. Exhaust everything above first.
Operator documentation: support.xbox.com. A note on sourcing. Microsoft's support site is a single-page application: any invented URL of the form error-code-anything returns HTTP 200 with an empty shell. So a link that looks like it works may lead nowhere, and the 0x prefix is genuinely inconsistent between articles. The URL above is the one that actually resolves for this code. No 0x prefix in the URL.
Microsoft gives four causes: an Xbox Live connectivity issue, wrong Microsoft account details, a problem with your Microsoft account itself, or too many accounts saved to the console.
This one genuinely goes both ways, so the first job is telling which you are looking at.
Start with: Check the Xbox status page. Covers cause one.