On mobile I get a helpful error message about I don't allow third party cookies. On desktop it fails silently. I know that devs behind it are doing their best. It must be quite difficult to build this simple-on-the-surface-but-regulatory-compliant-complex service on what is likely had a short deadline.
Technical nits aside, it does work as advertised. It took over a week after delivery but my ballot has been marked as received. Progress.