How to Fix Error 400 When Signing Into a Microsoft Account

If you’re getting Error 400 when signing into your Microsoft account, it usually appears as “Bad Request” or a vague sign-in failure message in the browser or an app. This error commonly shows up due to corrupted cookies, cached sign-in data, browser extensions, or account session conflicts—rather than a problem with your account itself.

The good news is that Error 400 is almost always fixable with a few targeted steps.

Fix Error 400 When Signing Into a Microsoft Account

The fixes below focus on clearing broken sign-in data and resetting the authentication flow. We recommend following them in order and retrying the sign-in after each fix, as the issue is often resolved early.

1. Clear Microsoft Sign-In Cookies and Cache

Corrupted cookies are the most common cause of Error 400 during sign-in. Clearing them forces the sign-in page to start fresh.

1. Open your browser settings.
2. Go to Privacy & security.
3. Clear cookies and cached images/files.
4. If available, target cookies for Microsoft-related sites only.
5. Close all browser windows.
6. Reopen the browser and try signing in again.

This removes broken session data that causes malformed requests.

2. Sign In Using a Private or Incognito Window

Private browsing disables extensions and uses a clean cookie session, which is ideal for testing sign-in issues.

1. Open a new Incognito / Private window in your browser.
2. Go to the Microsoft sign-in page.
3. Enter your account credentials and complete the sign-in.

If this works, the issue is almost certainly caused by cached data or extensions in your regular browser session.

3. Disable Browser Extensions Temporarily

Ad blockers, privacy tools, and script blockers can interfere with Microsoft’s authentication flow and trigger Error 400.

1. Open your browser’s extensions or add-ons page.
2. Disable all extensions temporarily.
3. Restart the browser.
4. Try signing in again.
5. Re-enable extensions one by one after successful sign-in.

Authentication pages are especially sensitive to blocked scripts and redirects.

4. Check Date, Time, and Time Zone Settings

Incorrect system time can break secure sign-in requests and cause authentication errors.

1. Open Settings on your PC.
2. Go to Time & language > Date & time.
3. Enable Set time automatically and Set time zone automatically.
4. Restart your browser and retry the sign-in.

Even small time mismatches can cause request validation to fail.

5. Try a Different Browser or Device

If the error persists, switching browsers helps rule out browser-specific corruption.

1. Open a different browser that you don’t normally use.
2. Go to the Microsoft sign-in page.
3. Attempt to sign in using the same account.

If it works in another browser or device, the problem is isolated to the original browser profile.

6. Remove Saved Microsoft Account Credentials

Saved passwords or corrupted sign-in tokens can cause repeated Error 400 loops.

1. Open your browser’s Password Manager.
2. Remove any saved entries related to Microsoft accounts.
3. Restart the browser.
4. Sign in manually by typing your email and password.

This ensures the browser doesn’t reuse a broken credential token.

7. Check Microsoft Service Status

Occasionally, Error 400 can be caused by temporary server-side issues.

1. Visit Microsoft’s service status page.
2. Check for ongoing authentication or account-related outages.
3. If services are down, wait and try again later.

Outages from Microsoft are rare, but they do happen.

Wrapping Up

Error 400 when signing into a Microsoft account is almost always caused by corrupted browser data, blocked scripts, or session conflicts—not a locked or hacked account. Clearing cookies, disabling extensions, and using a private window usually resolves the issue within minutes.

If the error keeps returning, switching browsers or removing saved credentials provides a clean authentication path. Once the sign-in completes successfully, you can return to your normal browser setup without further issues.