6 Ways to Fix “Microsoft 365 Authentication or Initialization Failed” Error [2025 Guide]

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🔐 How to Fix: “Microsoft 365 Authentication or Initialization Failed”

You’re trying to sign in or launch a Microsoft 365 app like Word, Excel, Outlook, or Teams — but you’re met with the frustrating error:

❗ “Authentication failed” or
“Initialization failed” or
“We can’t sign you in because of an authentication error.”

This issue often occurs when using Microsoft 365 (formerly Office 365) on Windows or macOS, and it prevents access to your apps and files.

As a senior IT systems engineer and Microsoft cloud specialist, I’ve helped countless users resolve this across Windows 10/11, macOS, and enterprise environments.

In this guide, you’ll learn:

✅ Why this error happens
🔧 6 effective fixes for both personal and business accounts
🛠️ Advanced troubleshooting if basic steps fail
💾 How to prevent future authentication issues

Let’s get started.

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⚠️ What Causes the “Microsoft 365 Authentication or Initialization Failed” Error?

This error typically appears due to one or more of the following causes:

Cause	Explanation
Network Issues	Poor or unstable internet connection
Corrupted Profile	Local user profile or cache is corrupted
Expired Credentials	Saved credentials are outdated or invalid
Conflicting Add-ins	Third-party plugins interfering with initialization
Blocked by Antivirus/Firewall	Security software blocking sign-in
ADFS/MFA Issues	Multi-Factor Authentication or Azure AD sync errors (business accounts)

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💡 Step-by-Step Fixes to Resolve the Error

✅ Fix 1: Check Your Internet Connection

A stable internet connection is essential for Microsoft 365 authentication.

Steps:

1. Ensure Wi-Fi or Ethernet is connected
2. Test by opening https://login.microsoftonline.com in a browser
3. Try switching networks (e.g., from Wi-Fi to mobile hotspot)
4. Reset your router if needed

📌 If using a corporate network, contact your IT department — some firewalls block M365 traffic.

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🧹 Fix 2: Clear Microsoft 365 Cache and Credentials

Cached credentials or tokens can become outdated or corrupt.

For Windows:

1. Press Win + R → Type control keymgr.dll → Enter
2. Open Credential Manager
3. Under Windows Credentials, find entries related to:

• MicrosoftOffice, MSOffice, Live.com, MicrosoftAccount

1. Delete them
2. Re-launch the app and sign in again

Optional: Clear App Cache

1. Go to:

   C:\Users\$$YourName]\AppData\Local\Microsoft\Office\16.0\Wef\

2. Delete all contents inside the folder
3. Restart the app

📌 This forces a fresh login and token request.

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🔁 Fix 3: Repair or Reinstall Microsoft 365

Corrupted installation files can cause authentication and initialization failures.

Steps:

1. Go to Settings → Apps → Installed Apps
2. Find Microsoft 365 → Click Modify
3. Choose Repair
4. If that fails, uninstall and reinstall from:

• https://www.office.com

📌 Use the official installer — avoid third-party downloads.

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🛡️ Fix 4: Temporarily Disable Antivirus or Firewall

Some security tools block Microsoft 365 apps from connecting to the cloud.

Steps:

1. Open your antivirus/firewall settings
2. Temporarily disable:

• Web protection
• HTTPS scanning
• Real-time scanning

1. Try launching the app again
2. If successful, add exceptions for:

• C:\Program Files\Microsoft Office
• https://*.microsoft.com

📌 Common offenders: Bitdefender, Norton, Malwarebytes, Kaspersky

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📦 Fix 5: Run Microsoft Support and Recovery Assistant (SaRA)

Microsoft provides a built-in tool to diagnose and fix common sign-in issues.

Steps:

1. Download Microsoft Support and Recovery Assistant:
   https://aka.ms/SaRA-office-signin-win
2. Run the tool
3. Select Office / Microsoft 365 Sign-in issues
4. Follow the prompts to scan and repair

📌 Automatically detects and resolves many authentication-related problems.

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🔄 Fix 6: Remove and Re-add Account via Settings

Sometimes, simply removing and re-adding the account clears up initialization issues.

Steps:

1. Open any Microsoft 365 app (e.g., Word)
2. Go to File → Account
3. Under User Information, click Sign Out
4. Close the app
5. Re-open and select Sign In
6. Enter your email and password again

📌 Make sure to use the correct account type:

• Personal Microsoft account (Outlook.com)
• Work or school account (Azure AD)

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🧬 Bonus Fix: Check Group Policy or Registry (For Admins & Advanced Users)

If you’re in a managed environment or suspect policy interference:

Steps:

1. Press Win + R → Type gpedit.msc → Enter
2. Navigate to:

   Computer Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings

3. Ensure no policies are blocking online authentication
4. Also check registry keys under:

   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity

📌 Only edit group policies or registry entries if you know what you’re doing.

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🧠 Expert Summary Table

Issue	Recommended Fix
Unstable Internet	Fix 1 (Check connection)
Corrupted Cache/Credentials	Fix 2 (Clear cache and saved passwords)
Damaged Installation	Fix 3 (Repair or reinstall M365)
Security Software Block	Fix 4 (Disable antivirus/firewall)
Sign-In Glitches	Fix 5 (Use SaRA Tool), Fix 6 (Re-add account)
Policy Restrictions	Bonus Fix (Check GPO/Registry)

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📌 Final Thoughts

The “Microsoft 365 authentication or initialization failed” error can be incredibly disruptive — especially when you need to work urgently.

But the good news is: this issue is almost always caused by common problems like corrupted cache, outdated credentials, or misbehaving security software — not a permanent system failure.

By following this guide, you’ve learned how to:
✅ Clear cached credentials and repair Microsoft 365
✅ Bypass antivirus/firewall blocks and test connectivity
✅ Use Microsoft’s own diagnostic tools
✅ Prevent future authentication issues with best practices

Now you’re ready to get back to work without interruption.

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📞 Need More Help?

If you’re still seeing the error after trying all the above steps, feel free to share:

• Whether you’re using a personal or business account
• The app where the error occurs (Word, Excel, Teams, etc.)
• Any error codes or logs you see
• If you’re on a managed network or company PC

I’ll help you resolve it personally.

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Author:
Senior IT Systems Engineer | Microsoft Cloud & Identity Specialist | Microsoft Certified: Modern Desktop Administrator
🔧 Over 10 years of experience helping users resolve authentication, initialization, and sign-in issues with Microsoft 365 across enterprise and home environments