Office Compatibility Issues

How to Fix Office Authentication Protocol Errors When Opening SharePoint Files

Kushani Nimanthika Sep 30, 2026 870 views

Question details

Users are encountering an error stating the server uses an unsupported authentication protocol when trying to open SharePoint files, often accompanied by repeated OfficeFileCache failures.

Fix Office Authentication Protocol Errors When Opening SharePoint Files

Product: Microsoft Office, SharePoint
Device & OS: not provided
Scenario: Opening SharePoint files from desktop Office applications.
Observed behavior: Office fails to open the file, displaying an authentication protocol error and experiencing OfficeFileCache sync failures.

Before you start

Ensure your internet connection is stable and save any open local documents before attempting to clear caches or reinstall Microsoft Office applications.

Solution 1Recommended

Perform a Complete Reinstallation of Office

Use Microsoft's official uninstall tool to deeply clean corrupted cache files and reinstall Office to resolve authentication protocol mismatches.

Download the Uninstall Tool

Download Microsoft's Support and Recovery Assistant (SaRA) or the official Office uninstall support tool from the Microsoft website.

Run the Uninstaller

Run the downloaded tool and follow the on-screen prompts to completely remove Office, which will clear all corrupted OfficeFileCache data and broken credential tokens.

Restart Your Computer

Once the tool finishes, restart your computer to finalize the removal process and clear system memory.

Reinstall Office

Log in to your Microsoft 365 portal, navigate to the app installation section, and download the latest version of Office to ensure modern authentication compatibility.

Perform a Complete Reinstallation of Office

Data Safety: Using the uninstall tool only removes application files and broken configurations. Your personal documents saved locally or on SharePoint remain completely safe.

Solution 2

Escalate to IT Admin or Microsoft Support

If a deep reinstallation fails, the issue may stem from backend server configuration requiring advanced diagnostic logs.

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Frequently Asked Questions

What does 'unsupported authentication protocol' mean in Office?

This error usually occurs when the Office desktop client is trying to use older or mismatched security protocols (like Basic Authentication) that your SharePoint server or Microsoft 365 organization has disabled in favor of Modern Authentication.

Why is the OfficeFileCache repeatedly failing?

The Office Document Cache (OfficeFileCache) can become corrupted if file syncing is interrupted by an authentication failure, leading to a loop where Office cannot upload or download the latest file changes from SharePoint.

Can clearing normal Office caches fix this problem?

Sometimes. You can try clearing the cache via the Office Upload Center or deleting the OfficeFileCache folder manually first. However, if the protocol error persists, a complete reinstall using the Microsoft tool is often necessary.

Office App Problems