Troubleshooting Office 365 login and authentication issues requires a systematic approach to identify and resolve common causes like corrupted credentials, misconfigured settings, or network-related problems. Most issues stem from either client-side factors (browser cache, outdated software, or credential conflicts) or server-side factors (authentication loops, MFA misconfigurations, or service outages). The most effective solutions include running Microsoft’s built-in troubleshooters, clearing cached credentials, verifying network settings, and ensuring proper configuration of multi-factor authentication (MFA) tools.

Key findings from the sources include:

• Microsoft 365 Sign-in Troubleshooter is the first recommended tool for diagnosing and automatically fixing common sign-in problems ^[1].
• Browser-related issues (cache, cookies, or extensions) frequently cause login loops, with incognito mode or clearing cache often resolving the problem ^[3][6].
• Credential conflicts in Windows Credential Manager or macOS Keychain can block authentication, requiring manual removal of stored Office credentials ^[2][7].
• Multi-factor authentication (MFA) misconfigurations, such as disabled authenticator apps or incorrect security policies, can trigger account locks or authentication failures ^[8][9].

Step-by-Step Troubleshooting for Office 365 Login and Authentication Issues

Basic Troubleshooting: Quick Fixes for Common Issues

Start with these steps to resolve the most frequent login problems before diving into advanced diagnostics.

For browser-based login issues (e.g., portal.office.com loops or errors):

• Use Incognito/InPrivate mode to bypass cached data or conflicting extensions. This isolates whether the issue is browser-specific ^[3][6].
• Clear the browser’s cache and cookies, as corrupted data can prevent successful authentication. In Chrome, navigate to Settings > Privacy and Security > Clear Browsing Data and select "Cookies and other site data" and "Cached images and files" ^[6].
• Try a different browser (e.g., Edge, Firefox) or device to rule out local configuration problems. If login works elsewhere, the issue is likely tied to the original browser or device ^[3].

For desktop app issues (e.g., Outlook, Word, Excel):

• Run the Microsoft 365 Sign-in Troubleshooter, a dedicated tool that automatically detects and repairs common authentication problems. Download it from Microsoft’s official support site or run it via PowerShell with the command:

Get-AppxPackage Microsoft.AAD.BrokerPlugin | Remove-AppxPackage

This reinstalls the Active Directory Authentication Library (ADAL) or Live ID components, which are critical for sign-in functionality ^[1].

• Sign out and back in with your Work or School account. Navigate to File > Account in any Office app, select "Sign Out," then re-enter your credentials ^[2].
• Perform an Online Repair of Microsoft 365 via the Control Panel: 1. Open Control Panel > Programs and Features. 2. Select Microsoft 365, click Change, then choose Online Repair ^[5][7].

For credential-related issues:

• Remove conflicting credentials from Windows Credential Manager: 1. Open Control Panel > User Accounts > Credential Manager. 2. Under Windows Credentials, remove any entries labeled MicrosoftOffice16_Data or MicrosoftAccount ^[2].
• On macOS, reset Keychain access if Office prompts for password repeatedly: 1. Open Keychain Access (via Spotlight or Applications > Utilities). 2. Search for "Office" or "Microsoft" and delete related entries ^[2].

Advanced Diagnostics: Network, MFA, and System-Level Fixes

When basic steps fail, investigate deeper system or network configurations.

Network and Proxy Settings:

• Ensure TLS 1.2 is enabled, as older protocols (TLS 1.0/1.1) are deprecated and may block authentication. On Windows: 1. Open Registry Editor (regedit). 2. Navigate to HKEYLOCALMACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols. 3. Verify that TLS 1.2 is enabled under both Client and Server subkeys ^[2].
• Check for proxy or firewall restrictions that may block Office 365 URLs. Ensure the following endpoints are whitelisted:
• https://login.microsoftonline.com
• https://outlook.office365.com
• https://*.microsoft.com

Test connectivity by pinging these URLs or using Test-NetConnection in PowerShell ^[2].

• Disable VPNs or corporate firewalls temporarily to isolate whether they interfere with authentication. Some VPNs route traffic through servers that lack access to Microsoft’s authentication services ^[6].

Multi-Factor Authentication (MFA) Issues:

• If prompted for Microsoft Authenticator despite already setting it up, the app may not be properly registered: 1. Open the Authenticator app and check if the account appears under "Work or School." 2. If missing, add the account manually by scanning a QR code from https://aka.ms/mfsetup ^[3]. 3. If the app is disabled, wait 30 minutes for the temporary lock to lift before retrying ^[8].
• For MFA enrollment errors (e.g., "There was a problem processing your request" with error code 0), check Microsoft’s Service Health Dashboard (https://admin.microsoft.com/AdminPortal/Home/servicehealth) for outages. Widespread MFA failures often indicate a server-side issue ^[10].
• If your organization suddenly requires MFA without prior setup, contact your global admin to verify security policy changes. Admins can adjust MFA requirements via the Azure AD Portal (https://portal.azure.com) ^[9].

System-Level Conflicts:

• Antivirus or security software may block the BrokerPlugin process, which handles authentication. Temporarily disable the antivirus and test login. If successful, add an exception for:
• C:\Program Files\Microsoft Office\root\Office16\lync.exe
• C:\Program Files\Microsoft Office\root\Office16\msoia.exe ^[2].
• For Virtual Desktop Infrastructure (VDI) or Citrix environments, ensure Microsoft 365 is configured for shared activation. Use the Office Deployment Tool to apply the correct licensing mode:

• Verify Date and Time settings are synchronized with a Network Time Protocol (NTP) server. Incorrect time settings can cause authentication tokens to fail. On Windows: 1. Right-click the clock > Adjust date and time. 2. Enable Set time automatically and select time.windows.com as the server ^[6].