Can't start the mod from within eclipse

[obscuremusic]

I am trying to start the "mod" I made from within the eclipse editor. The problem is, that I alway get this error:

 

[13:15:02] [main/INFO] [GradleStart]: Extra: []
Exception in thread "main" java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
    at net.minecraftforge.gradle.GradleForgeHacks.searchCoremods(GradleForgeHacks.java:85)
    at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:86)
    at GradleStart.main(GradleStart.java:25)

 

I am extremely new to modding and I haven't found anything that could help me with this.

Please let me know if you have an idea of how to fix this.