@gbengeult I managed to fix things, but only because I do daily backups. I deleted my entire Opera configuration and then replaced it with the backup from one day before the Opera One update. Then I opened Opera One and everything came up correctly. I use linux, so my configuration directory is at /home/<user>/.config/opera.
On an unrelated note, my Opera configuration directory is over 1.2 GB!! What on earth could cause this browser to need that much disk space?