I got very strange behavior that has never happened before, when I try to configure the GitHub server in Jenkins general configuration to set up webhooks auto.
The d
The issue is that the GitHub plugin only accepts plain text credentials.
The GitHub access token can be created manually, or automatically via the Advanced... options as described here.
In case you already have an access token in GitHub (you'll get an error in Jenkins), you can remove it in Github. Then you can let Jenkins generate the token and select it in the Credentials menu.