One of your mods is using the random number generator incorrectly.
The error message does not identify which one.
This is what I found when searching github for similar issues: https://github.com/search?q=Accessing+LegacyRandomSource+from+multiple+threads&type=issues
Maybe one of them matches a mod you are using?
You are correct, it is a java question.
You appear to be using an unsupported (in this forum) version of forge, I think 1.12? But then trying to use api that is from forge 1.19, e.g.
import net.minecraftforge.fml.common.eventhandler.SubscribeEvent; // <-- 1.12
import net.minecraftforge.eventbus.api.SubscribeEvent; // <-- 1.19
RegisterEvent was introduced in 1.19
It seems that you moved the files, but you haven't refreshed your launch files for your IDE.
Remove the old launch files, then run:
For Eclipse: gradlew genEclipseRuns
For IDEA: gradlew genIntellijRuns
For VS Code: gradlew genVSCodeRuns