On November 16, 2020 a 504 Gateway was reported with ClassLink. They are working to resolve the problem. This post will be updated when the problem is resolved.
Update: 8:40 November 16, 2020
Issue resolved. Users may need to refresh on sign-in.
Update: 8:20 am (PST)
504 Gateway Time-Out Error
AWS engineers continue to investigate. Our internal team believes it's related to internal network corruption at AWS and is proactively reprovisioning all front end servers.