Same with my server. I first saw a Connect error that it cannot connect to mothership.unraid.net. I signed out of the account which succeeded, but I can no longer sign in. Same error as reported above.
Edit: Also the Management Access page takes a really long time to load. A reboot fixed the loading time. But Sign In to Connect continues to fail.
Edit 2: Turns out there was an issue with my ipv6 assignment. Once I fixed that, sign in worked immediately.