Hey, you have a Mixin issue.
It is not Sponge related so we cannot help you. A mod is just using something called Mixins which while is made by Sponge, it is up to the mod to support it and use it correctly.
Sadly your log does not state what mod is causing the mixin error so it becomes very hard to work out exactly what mod is causing the issue.
I do have a program that will help you track down the mod causing it, the mod is called Mixin detector that will show you all the mods that have mixins. it will be one of those mods.