We have a Flow that's connected to/triggered from a PowerApp. Students use the App to submit proposals for review and approval. Last Fall was our first term using the App and students had zero problems. However, this term, so far two students (who submitted successfully in the Fall) have had the Flow fail when trying to send them an email using the Office 365 Outlook Send Mail action. The exact error message text is:
Error from token exchange: Runtime call was blocked because connection has error status: Enabled| Error, and office365 is in the block list. Connection errors: [ParameterName: token, Error: Code: Unauthorized, Message: 'Failed to refresh access token for service: office365certificate. Correlation Id=606df5da-5e8a-4b1a-bb4c-c99955c87bfa, UTC TimeStamp=1/17/2019 3:07:11 AM, Error: Failed to acquire token from AAD: AADSTS70002: Error validating credentials. AADSTS700082: The refresh token has expired due to inactivity. The token was issued on 2018-10-12T18:34:05.7604799Z and was inactive for 90.00:00:00.
Trace ID: 7bc450f9-9af8-4a13-8fa5-b4e799960700
Correlation ID: 3ab34416-61f3-4402-bf67-5a09e719d668
Timestamp: 2019-01-17 03:07:11Z']
What appears to be happening is that the connection token (which they "allowed" when running the App back in the Fall) expired/timed out and did not refresh when they ran the App this term. This has only happened to two students thus far. Dozens of others have used the App/Flow and submitted proposals without error.
For the time being, I added another action (the generic "Mail" one that sends from MS's service account) that runs only if the O365 Outlook Send Mail fails. In that message, I direct them to check their Connections in Flow (which seemed to fix the issue for the first student, still waiting to hear back from the second one).