I honestly don't know wich of the mods i have is making this happen, when i check the technology it unlocks an Assembling Machine 2 from the mod, on the list of all the mods that change the assembler are: Base mod > Krastorio 2 > AAI Industry > Combat Mechanics Overhaul > Space Exploration > Bottleneck Lite > Base mod > Space Exploration, don't know if it's because of any of these

I've confirmed this is specifically a conflict with AAI Industry - tested with no other mods except these two.

This seems to be because AAI Industry replaces single results in recipes with result lists, and this mod isn't prepared to deal with that when creating the refurbisher recipe.

Since Mylon doesn't seem to have been active for some time, I created a fixed version of the mod: https://mods.factorio.com/mod/Assembler_Assay_fixed

Mylon, if you're reading this, let me know when you've fixed the issue in your mod and I will mark my version as deprecated.

This is fixed. Sorry for the long wait.