Sponge allows the ability to run sponge plugins on a server.
The sponge team created something called Mixins which allows any mod to hook directly into minecraft. Sadly not all mods like Mixins being present, sometimes the mod developer creates a bug when using mixins and a few other reasons that may cause a mixin crash.
This is more common and has caused a lot of forum posts where server admins come to us asking for a solution (thank you for not blaming sponge as its not sponge’s fault)
Because its so common ive created a program known as Mixin Detector which detects all mods that have mixins. This will cut down your 200+ mods down. It also has a filter option. If you filter by file and then provide create.mixins.json (the file that keeps showing in your log that you provided). This should show you what mod is causing you an issue.
From there you can ask the mod developer(s) for help, just remove it or something else.
Hope that helps