You should only get this error if paired with S2 security. It is a bug in the Firmware Updater app that does not seem like it is ever going to be fixed. If it is paired with no security it will work correctly, as I have updated two of them myself using the app.
Also, as you have stated, I do not think there ever was a 1.24 firmware for that device. So now I am skeptical if this is even a ZEN51, and/or if it is still paired with S2.
Can you please post two screenshots, one of the "Data" section at the bottom of the device page. And another of the Settings > Zwave details page (just the row for this device).
Examples (deviceModel and configVals will only be present if you use my custom driver):
Driver: [DRIVER] Zooz Relays Advanced (ZEN16, ZEN17, ZEN51, ZEN52)
