We are also seeing this in beta 5 still. Over Github actions it succeeds sometimes and fails other times, but no build ever fails when run on the runner in command line manually.

Having the same issue. This is clearly an issue the App Review team needs to address. We have about a quarter million daily users and testers in 3 different timezones working from home and no one has this issue not being able to sign in. This also never happened until Apple (and everyone else) started doing work from home. It's something with Apple's networking stack. This user has a bit more detailed of a response from their debugging: https://developer.apple.com/forums/thread/653565