Yesterday I had the same problem on my Xperia 10 II, unlock with fingerprint was not possible. I took a look in the settings, no fingerprint showed up. Reboot did not change anything. I added one finger, tried to add another but the limit of 5 was reached. After the message telling me I have reached the limit, all 5 fingerprints showed up again in the list. I deleted the last created duplicate, but unlock with fingerprint did not work. Another reboot and finally unlock via fingerprint is working again (and they still show up in the list).
That’s a strange error and neither rebooting nor adding fingerprints always fix it.