We are aware of an ongoing issue with WDQS that begun around 1500 UTC today. To stabilize the service, we have introduced drastic measures including banning cloud provider IPs and certain user agents. Unfortunately, this will likely affect legitimate users of the service. We are currently working on less disruptive measures and will update https://phabricator.wikimedia.org/T324346 when we have more information to share. Thanks for your patience, and we sincerely apologize for the inconvenience.