Armor not working

i'm playing in a server with some friends and for some reason the armor is not working, you can wield it but it doesn't protect from damage, and is not showing, any solution?

This started to happen when i tried to install a mod, the server was vanilla and after that mod it switched to forge and can't switch back to vanilla, the mod is uninstalled.

Quote from Toothy

Download your world, switch to vanilla and upload it again, all after doing a backup. Then try again

When i switch to vanilla and upload the world again it switches to forge by itself