Hi @LukasFrey, I am very sorry you are having trouble. When testing the OAuth auth flow on my own n8n instance against MS365, I was able to see the full error with the default log level settings:
This was using n8n v0.236.0. Perhaps you can try upgrading your current version of n8n and verify whether the issue still persists? If so, you could try increasing the log level and take a peek at the server logs, though OAuth logging in n8n is unfortunately rather limited.
Also, can you confirm which credential type exactly you are using? Are these n8n’s pre-configured OAuth2 credentials for Microsoft Outlook or different credentials such as generic OAuth2 credentials (like here)?