Whenever I try to launch the modpack I have created, it crashes before reaching the main menu.

Here is latest log.
List of mods I have installed here, but when I disable ModernFix, it loads just fine.

One of your other mods loads a Minecraft class too early, which is bad practice and causes compatibility issues with other mods (like ModernFix). Please try to isolate which other mod is the culprit.

As a workaround you can add mixin.perf.thread_priorities=false at the bottom of config/modernfix-mixins.properties.

Apparently Rubidium 0.7.0 breaks this. It will need to be fixed on its end. I'll pin this issue so people coming to the tracker can see it.