My mistake, overlooked, already fixed.

It's this mod. In your food's itemtypes, you have it transitioning into "rot" instead of "game:rot". There's no "moremeat:rot" for the game to target, so it crashes when it tries to transition into something that cannot exist.

And moreover, almost completely identical written functions to those in an identical standard game item.

Most likely this is not a problem of the mod, since it uses standard functions that are also used in the game itself.

Mod caused an error and I had to remove it from my server, had some mutton in a players inventory reach 100% rot, it was unable to enter into the state of being rot, meaning whenever that player connected the game would try and make it rot, be unable to do so, and crash the server.

I haven't added it yet

I can't add mutton to any stews.

Сhicken replaces the standard one when harvested(a little more satisfying), with further crafting to the standard one and mutton with nothing but the name and color)

What's the difference between the new meat and the old one?