On ATM7 we now use MixinBootstrap so it trys to load some mixin stuff in your mod that is not updated yet.
https://hastepaste.com/view/6Gbd7
https://www.curseforge.com/minecraft/mc-mods/mixinbootstrap/files/3437402

Just released a new build this issue should be resolved but note that MixinBootstrap should no longer be needed as of latest version of Forge