The issue has been fixed by the Notion team -- https://status.notion.so/incidents/ch2cmpln7jjg

We have identified the issue to be with the https://api.notion.com/v1/oauth/token endpoint. Other products providing Notion integrations are also facing the same issue. We are in touch with the Notion team to understand more.

Notion's OAuth flow is failing at access token generation. Investigation ongoing.