I wanted to start a project with some friends, with a small mod list. It includes mods that need a specific version of 1.12.2 Forge because of render changes. Also, our server hosting site only accepts One of the few Forge versions the mods need to work - and if I Start this version (forge 14.23.5.2768) The game shows me that it needs a newer version of Forge. I tried every on curse forge exciting jei version, and it still doesn't work.
Here a screenshot of the error message - hope it helps

Could you please help me with that? I really need the mod in the project.

If I'm reading things correctly, the first build for Forge 14.23.5 supported by JEI is 2775. I suppose you could try going back into 14.23.4, but now you're in breaking-changes territory, and the odds of some other mod not working goes up sharply.