On screen: 0x8027025a or 8027025a
This one genuinely goes both ways, so the first job is telling which you are looking at.
Microsoft gives three causes explicitly: there's a temporary problem with the Xbox service, the application did not load in the time expected, or there's a sign-in issue with your profile.
That is unusually helpful, because the three are separable and the tests are quick.
Three checks. If it happens in every app and on more than one console, it is the service. If it is one specific app, that app timed out loading. If a different profile signs in fine on the same console, it is your profile.
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 for this one, despite the article title using it.
Microsoft gives three causes explicitly: there's a temporary problem with the Xbox service, the application did not load in the time expected, or there's a sign-in issue with your profile.
This one genuinely goes both ways, so the first job is telling which you are looking at.
Start with: Check the Xbox status page. Microsoft's first step, covering cause one.