I see there is currently one GitHub team is configured under dateutil AppVeyor account. It's called dateutil/Owners and mapped to Administrator role in AppVeyor.
You are probably not the member of dateutil/Owners GitHub team (has your membership there changed recently?).
To restore your access to dateutil account please ask account owner "jarondl" ([email blocked]) to either make you the member of dateutil/Owners GH team (and next time you re-login with GitHub button you will re-gain access to dateutil account) or create a new GH team, e.g. dateutil/Devs, map it to a different role in AppVeyor (https://www.appveyor.com/docs/team-setup/#github-integration).
I deleted that another dateutil33581 account you just created.