Troubleshooting: "Sync Error" in MTR Calendar

Modified on Wed, 1 Apr at 1:13 PM

KB: Troubleshooting MTR Sync Error 500401 & Infinite Loops

1. Problem Identification

A specific resource mailbox displays Sync error while others function normally. The diagnostic log typically shows:

Error 500401: Organization token unauthorized

2. Root Cause Analysis

In a scenario where only one room fails, Error 500401 is often a side effect. The primary triggers are:

Authentication Failure: Expired password, disabled account, or missing Teams Room Pro/Basic license.
Graph API Throttling: A corrupted calendar item causes a "Delta Query Infinite Loop," leading Microsoft to revoke the token.

3. Action Plan (Priority Order)

Phase 1: Account Validation

Verify account is Enabled and has a M365 License.
Log in to OWA. If OWA fails, sync fails.

Phase 2: Data Sanitization (CalCheck)

Run CalCheck tool.
Delete items marked "Critical Error".
Watch for recurring meetings without end dates or large attachments (>1MB).

Reference: Microsoft Graph API Documentation