In 1.20.4 game verstion it can't find main class file(any class file to be exact). The implementation of the class:

cpw.mods.jarhandling.impl.Jar

differs in 1.20.1 and 1.20.4. As a result it returns different paths and the loader searches for location where are no classes. In case I manualy copy classes to \build\sourcesSets\main it works.