Sadly mixins while created by sponge, isnt something we are responsible for (unless its in sponge). Instead a mod is using it and it is responsible for it.
Your log doesnt seem to be complete and therefore I cannot see the mixin file that is causing the error so I wont be able to tell you the exact mod causing it.
With that being said, i can filter down your mods list to those mods with mixins. I have made a program that will filter down your mods to those with mixins. It has filters that can be applied too if you know things to filter by.