error while loading item prototype "down-belt" (item):Key "icon" not found in property tree at ROOT.item.down-belt.icons[0]
Modifications: 3D Belts

Interesting, I cannot replicate this error.

What version of Factorio are you running, and are there any other mods?

I assume this happens at startup?

yep, on startup, im running the current version with a large mod list

I see... apparently there is a mod that changes the icon structure of vanilla belts, since I just copy the icon from the base belts.
Do you mind sending a mod list so I can try and trace it down?

oh, i think i know what it might be: belt reskin. and now i have black rubber belts in it's place. i'll try it without either
edit: it seems incompatible with stacked even though it doesn't change anything but stack sizes
edit 2: hold on, it's artisanal reskins doing it

ok, so it is artisanal reskins causing the crash. at least it has settings to disable the belt changes
edit: ok so it's just entirely incompatible, unfortunately
edit 2, electric boogaloo: stacked is compatible. have no idea why it was taking that down with it, but, oh well
¯-(ツ)-¯

0.0.5 has been published. It works with Artisanal Reskins, turns out I'm bad at modding and I was creating the new belts in the wrong place. It should be much more compatible now.