SDErrorDomain error 108

Hello, on a Mini M1

I have tried installing Monterey (12.1) on an external SSD (SanDisk Extreme 1Tb); installation went ok.

However:

1) if I boot from the internal disk (with BigSur 11.6.2) and try to set the startup disk to the external one, I get the aforementioned error.

2) If I boot pressing the power button so that I can choose the boot disk, I can boot from the external; once I have booted it, I can set the startup disk to be the external (or the internal as I wish).

What gives?

Also, a first attempt at installing from internal onto the external produced a non-bootable (in any scenario) installation.

I have since erased and reinstalled (from a bootable install disk also external) from scratch and I landed in the above scenario.