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Hello

I am trying to define a specific MAC address for one NIC and the desired MAC starts with 01:xxx

Apparently the GUI will not let me enter such a value (simply ignores the entry).

Any idea ?

Correct. The GUI also gives me a message to say that the second digit must be even.

Modify it through the Guest itself. Most OSes allow you to specify a MAC address to be used other than the hardware defined one.