Q: What does a 401 Unauthorized error mean and how is it different from a 403? A: A 401 Unauthorized error means the site or API cannot verify who you are because your request lacked valid credentials or the credentials were rejected. It differs from a 403 Forbidden, which means the server knows who you are but you do not have permission to access the resource. Q: What quick checks should I try first to learn how to fix 401 unauthorized error? A: Start by logging out and signing in again, clearing site cookies or using a private window, and confirming your email and password including Caps Lock and keyboard layout. Also double-check the URL or API endpoint, turn off VPN or proxy, and set your device time to automatic so tokens and sessions validate correctly. Q: How should developers format Authorization headers to avoid a 401 when calling an API? A: Use the exact scheme the API requires, for example Authorization: Bearer YOUR_TOKEN for OAuth/JWT, Basic base64(username:password) for HTTP Basic, or x-api-key for API keys. Be careful to include the Bearer prefix and correct spacing, and check token validity, scopes, audience, issuer, and signature when using JWTs. Q: Why might cookies cause a 401 and how can I fix cookie-related issues? A: Cookies can be blocked by browser privacy settings or SameSite rules so the server does not receive required session cookies, which can produce a 401. Fix this by clearing site cookies, testing in a private window, and ensuring cookies are set with SameSite=None; Secure, correct domain and path, and sent over HTTPS. Q: Can incorrect device or server time cause a 401 and what should I do about it? A: Yes, a bad device or server clock can break time-based tokens and OTPs and trigger 401 errors. Enable automatic time and time zone on your device and run NTP or check clock skew on servers to resolve the issue. Q: What should I do if a 401 only occurs on one network or behind a VPN? A: If a 401 happens only on one network, disable VPN or proxy and try a different network because those can hide or strip authentication information. Work with IT to whitelist the service, remove proxies that strip headers, and check gateway or CDN rules that might be stripping the Authorization header. Q: How can I prevent 401 errors from recurring? A: Prevent 401s by implementing silent token refresh, using a password manager to avoid credential typos, and keeping device and server time in sync with NTP. Also document required headers and scopes, return clear 401 messages with WWW-Authenticate for clients, monitor auth error rates, and rotate keys with an overlap period. Q: What information should I provide to support when a 401 persists? A: Share the time of the error and your time zone, the full URL or endpoint, request method, and any request ID from the response, along with whether you were logged in and the steps you tried. Also report whether it works on another network, browser, or device and include response headers, especially WWW-Authenticate, to help diagnosis.