This indicates the github oauth2 access token is being revoked or is no longer valid. These are the only known root causes for the issue that you have described:
- the user that activates the repository has their repository (or organization) access revoked. Drone clones the repository on behalf of the user that activates it [1]. If the user no longer has access, Drone will be unable to clone the repository.
- the user that activates the repository revokes Drone’s oauth application authorization (via the github settings screen)
[1] Does Slack plugin still support status: change? - #2 by ArturGajowy