@Michael E. Gernaey I'm hitting the exact same error when trying to list the MS Booking Pages in Power Automate.
The full error reads:
```
The dynamic invocation request failed with error: "Failed to get onbehalfof access token: connectionId=microsoftbookings, message={\"error\":\"invalid_grant\",\"error_description\":\"AADSTS50013: Assertion failed signature validation. [Reason - Key was found, but use of the key to verify the signature failed., Thumbprint of key used by client: 'B0CD7xxxxxxxxxxxxxxxxx4E40A', Found key 'Start=01/25/2026 21:03:48, End=01/25/2031 21:03:48', Please visit the Azure Portal, Graph Explorer or directly use MS Graph to see configured keys for app Id '00000000-0000-0000-0000-000000000000'. Review the documentation at https://docs.microsoft.com/en-us/graph/deployments to determine the corresponding service endpoint and https://docs.microsoft.com/en-us/graph/api/application-get?view=graph-rest-1.0&tabs=http to build a query request URL, such as 'https://graph.microsoft.com/beta/applications/00000000-0000-0000-0000-000000000000']. Trace ID: 48b1432d-5d06-4abb-93b2-34c2b3e11300 Correlation ID: 489d18ce-c5b3-43cc-877b-ad7bd4f8473f Timestamp: 2026-02-25 20:27:59Z\",\"error_codes\":[50013],\"timestamp\":\"2026-02-25 20:27:59Z\",\"trace_id\":\"48b1432d-5d06-4abb-93b2-34c2b3e11300\",\"correlation_id\":\"489d18ce-c5b3-43cc-877b-ad7bd4f8473f\",\"error_uri\":\"https://login.windows.net/error?code=50013\"}".
```
I'd like to go in App Registrations as you suggested, but the App Id reads '00000000-0000-0000-0000-000000000000'. That reaaaaalllly looks like an invalid ID ;)
I'm on Power Automate Free to test it out. It's my first workflow, I tried deleting and recreating the MS Booking connection, same error. I've got the "Global Administrator" role in Azure / Bookings / etc., so I should be able to debug this if I knew what / where to look at.
Any ideas? Thanks!
