throw new Error("Unknown UI mode"); - when selecting Vertical Clock as default clock.

Argh, sorry about that. The odd thing is that the bootloader (via bootupdate.js) does define clockupdown. What firmware version do you have? process.env.COMMIT

I think what might have happened is your firmware is new enough to have Bangle.setUI but not new enough to have clockupdown in it. If you had an older firmware without it then the bootloader code would have 'polyfilled' it with the correct version