Mac OS 14 has a very nasty bug in Apple’s Core MIDI USB implementation.
The bug means that Apple can’t reliably transfer large (> ~60k) SysEx messages, which of course the Digitakt II needs.
There is no fix, and pretty sure Apple will never fix it. It is fixed in OS 15.
elk-herd won’t let you transfer DT 1 projects to DT 2. That has to be done with Transfer. Transfer will move the DT 1 project to a +Drive project slot… then when you load that project on the DT 2, the instrument will convert it.