The web GUI showed both disks (SSD boot disk and WD-Red data disk) were in pool "boot pool".
That was not my intention, the boot pool should only contain the SSD.
I tried to detach it via: "System | Boot | Actions | Boot Pool Status" but detaching failed.
Since I am just setting up a new system I rebooted Odroid and tried to do a fresh install of TrueNAS from usb stick on the correct disk.
So far so good, but I run into the same error: "The web interface could not be accessed. Please check network configration."
If I go to the shell and run "midclt call tunable.query | jq" the output shows the correct settings from HomeyBadger.
I'm very confused: these settings worked fine just before.
Any clues?
I ended up just reinstalling the system, at least that worked out fine.