In the absence of the ultra fast belt research (given that it has the same speed as the superior belt in K2) the faster belts can be researched, but cannot be manufactured anymore because the ultra-fast belt (and accessories - underground, splitter and loader) ARE REQUIRED by the subsequent faster belts. The only fix is to disable/remove this mod from the game.

Hello,
This mod changes the recipe of "Extreme fast belt" to need "superieur belt" in place of the "Ultra-fast belt". ( the Ultra-fast tier is complety gone from any recipe)

So if you still see the need for "Ultra-fast belt" that means there is possibly an other mod overwriting mine. (my theory)

I have checked the mod list and I tried with just "Ultimate Belts" and your K2 patch. It solves the problem indeed.

Apparently the main cause was the use of mod UltimateBeltsSmartFix 0.1.0 https://mods.factorio.com/mod/UltimateBeltsSmartFix. Once I removed this one, everything went just fine. :-)

Maybe flagging that mod as "incompatible" could prevent such situations. What do you think about this?

Merci beaucoup!

strange I tried this mod in a clean install but do not get the same result.
The recipe stays the same.
Also looking at the code it should not really affect the recipe has its disable.

(adding the code from that mod into mine would be somewhat easy too)

pas trouble.