It all stopped working this morning.
Try to assign internal testers:
POST https://appstoreconnect.apple.com/iris/v1/bulkBetaTesterAssignments 500 (Internal Server Error)
Try to open the test flight page for my app: GET https://appstoreconnect.apple.com/iris/v1/builds?filter[betaGroups]=87bbbfd2-5b5b-4397-b352-e5b395e2389c&filter[app]=6468433420&filter[expired]=false&filter[processingState]=VALID&sort=-version&limit=10&include=buildBetaDetail,betaBuildLocalizations,preReleaseVersion,betaAppReviewSubmission,icons,buildBundles,betaGroups,ciBuildGroup&fields[ciBuildGroups]=&limit[betaBuildLocalizations]=40&limit[betaGroups]=300 500 (Internal Server Error)