Ultracube fails to load on latest experimental versions, probably due to this change in 2.0.25:
"Added LuaBurnerPrototype::initial_fuel and initial_fuel_percent read"

Error:
Failed to load mods: Error while running setup for entity prototype "burner-inserter" (inserter): initial_fuel (wood) fuel_category (cube-fuel-general) is not compatible with this burner's fuel categories.

I don't know anything about modding but you can work around this by adding this line to <Ultracube Mod Folder>\prototypes\entities\inserters.lua:
data.raw.inserter["burner-inserter"].energy_source.initial_fuel=""

Another option to fix it is adding this line instead:

data.raw.inserter["burner-inserter"].energy_source.fuel_categories = {"cube-fuel-general"}

thanks for reporting, fixed on 0.6.4