[LAU] Behringer UMC1820 ADAT madness (not all channels working on SOME Laptops)

Hi, I have a Behringer UMC1820 that served me well in many multi-channel concerts I played. Now I need a few more channels, so I borrowed an ADAT expander (a re-purpused MOTU Traveler mkIII Firewire interface). I tried with two laptops, one Lenovo IdeaPad Flex 5 (11th gen Intel I7) and one Lenovo ThinkPad T14 Gen 3 (12th gen Intel I7). Other than that, both laptops have the same configuration. Arch Linux (updated today), Pipewire, Wireplumber, all drop-in replacements for pulse, Jack, etc, down to the same pipewire.conf (I copied that over). Here's the weirdness: On the ThinkPad, all ADAT output channels of the Behringer work, I can see the lights blinking and hear the sound. On the IdeaPad, only ADAT output channels 5-8 work, channels 1-4 are dead. I've tried different USB ports and so one, but nothing made a difference. An ideas where I could look? Could there be a setting I missed? Best, N