Most Teams sign-in problems fall into one of four groups: the wrong account or organization is being used, the browser is blocking the sign-in flow, the desktop app is holding onto stale local data, or an admin-side policy is stopping authentication. The fastest way to fix it is figuring out which group applies, rather than trying every fix at once.
Step 1: test on the web version first
Before troubleshooting the desktop app, try signing in at teams.microsoft.com in a browser. This single test tells you a lot:
- If the web version works but the desktop app doesn't — your account is fine, and the problem is local to the desktop app (stale cache, corrupted local state).
- If the web version also fails or loops back to sign-in — the issue is more likely with your account, browser cookie settings, or an organization-side policy.
Common error codes and what they mean
- 0xCAA90018 — Incorrect credentials. Double-check your email and password; if you're certain they're correct, your account password may have been recently reset by an admin without you knowing.
- 0xCAA20004 — An Azure Active Directory (Entra ID) configuration issue on your organization's side. This isn't something you can fix locally — contact your IT admin directly with this code.
- 0xCAA82EE7 / 0xCAA82EE2 — Often caused by a firewall, antivirus, or proxy blocking Teams' connection. Try temporarily disabling third-party antivirus/firewall software to test (re-enable afterward) or ask IT whether a corporate firewall rule might be blocking Teams' authentication endpoints.
- CAA2000B — Frequently resolved by running Teams as administrator, or relates to a system date/time mismatch (an incorrect clock can break the authentication handshake).
Fix: clear desktop app cache (most common fix)
- Fully close Teams
- Press Windows key + R, type
%appdata%\Microsoft\Teams, press Enter - Select everything in the folder and delete it
- Reopen Teams and sign in fresh
If you're on the newer Teams client (rebuilt architecture, default since 2023), try %localappdata%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache instead if the path above doesn't exist.
Fix: clear cached credentials
- Search for and open Credential Manager
- Click Windows Credentials
- Look under Generic Credentials for any entries related to Teams or your Microsoft account
- Remove them, then restart Teams and sign in again
Fix: check system date and time
An incorrect system clock can silently break the authentication handshake with Microsoft's servers — worth checking even though it seems unrelated:
- Go to Settings → Time & language → Date & time
- Make sure Set time automatically is on, and the time zone is correct
Fix: check for proxy or firewall interference
If you're getting connection-related error codes specifically, check whether a proxy is enabled (Settings → Network & internet → Proxy) and try disabling it temporarily, or ask IT whether Teams' authentication endpoints are allowed through your network's firewall.
Multiple accounts: a common source of confusion
Modern Teams supports signing into multiple work, school, and personal accounts simultaneously, and also signing in as a guest in other organizations. This flexibility is genuinely useful but is also a common source of "couldn't sign in" complaints that are really just the wrong account being used for a particular context — double-check which account you're actually signed into if you're being asked to authenticate unexpectedly when joining a specific team or organization.
If nothing above resolves it
If you've tried the relevant fixes for your specific error code and it persists, Microsoft provides an official Teams Sign-in Diagnostic tool (searchable as "Teams sign-in diagnostic" on Microsoft Learn) that can be run and the results shared with your IT admin — useful for narrowing down admin-side or Entra ID issues that aren't fixable from your end.