1. CHECK YOUR CONNECTION STATUS
Go to Power Automate > Data > Connections and look at your Office 365 Outlook and OneDrive connections. If you see a "Sign in required" warning between runs, that confirms the token is expiring. Re-authenticating manually each time is just a workaround, not a fix.
2. ROOT CAUSE — TOKEN REFRESH BLOCKED
This usually happens when:
- Your tenant's Conditional Access or MFA policy requires re-authentication after a certain period
- The account password was recently rotated or session was revoked
- The flow connection was created under a personal account rather than a service account
3. RECOMMENDED FIX — USE A SERVICE ACCOUNT
For unattended automation running on a schedule, always use a dedicated service account (shared mailbox or a dedicated M365 account) with:
- No MFA or exempted from Conditional Access for automation scenarios
- A stable password that doesn't expire
This prevents the session lifetime issue entirely.
4. ADD A RETRY POLICY AS A SAFETY NET
On the failing action, go to Settings and set Retry Policy to Exponential with 4 retries. This handles any transient failures while you fix the root cause.
5. CHECK THE DETAILED ERROR
Open the failing run > expand the failed action > look for "x-ms-failure-cause" or "invalid_grant" in the error body. If you see AuthenticationFailed or invalid_grant, it confirms the token issue and moving to a service account will fix it permanently.
Hope this helps! Let me know if the error body shows something different and we can narrow it down further.
