This seems to happen when the backup is encrypted. Unfortunately the spinner keeps spinning and only in the console / devtools of the browser you will see why. If you show the Response for this 400, you will see something according the lines of “Password wrong”. Even though it is not.
My workaround is to just set it up using a dummy user and then after that do a full restore from System > Settings > Backups. That works even with encrypted backups.