SD Card Mirroring fails

I’Ve just recently installed OpenHAB 3 on a Raspberry PI 3B+. All setup is working properly. So to be on the safe side, I bought another 16GiB SD Card and made a one time copy of the running SD card via openhabian-config menu option 55.
In addition, I wanted to setup SD mirroring with another 64GiB SD card. However, if I start the process via openhabian-config menu option 53, it runs for a bit and returns an error that the process could not be completed.

Can anyone help me on debugging that? I do not know where to start. Thanks!

What about reading the docs? There’s even a debug guide.