OAuth2 stopped working after several weeks reliable running

Questions
1

Describe the problem/error/question

I had two workflows which were both working then after 30 November 2025 stopped. The error is "Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method).
The first pushes data to an Excel 365 file in my OneDrive, the second emails notification using Outlook. Below is the Excel node as an example - the problem appears to be the same for both. NB I have replaced anything that looks like an ID or key with [RemovedForSecurity].
The AI help suggests that I need to use Azure platform to create a secret, however there doesn't appear to be anywhere to add that in n8n

What is the error message (if any)?

"Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method"

workflow to push data to Excel 365:

Share the output returned by the last node

There is no ouput from the node

Information on your n8n setup

  • n8n version: - Have updated to [email protected], but problem persists.
  • Database (default: SQLite):
  • n8n EXECUTIONS_PROCESS setting (default: own, main):
  • Running n8n via n8n cloud: starter plan
  • Operating system:

I’m not a programmer, so the OAuth2 stuff is beyond my knowledge - can anyone help with what I should do to get my workflow working again?

Thanks in advance.

2

Hey, this sounds like your OAuth2 tokens have expired and need to be reconnected. Microsoft OAuth credentials typically have a refresh token. What you’ll want to do is go back to your Microsoft Excel 365 and Outlook credentials in n8n and reconnect them by clicking the reconnect or reauthenticate button, which should let you refresh them!

3

Thanks but I tried that already and it made no difference.

4

Can you delete your current Oauth2, and make it again, and see if that works?