Problems while using mods in forge server

After installing the Forgeserver, there are two files. There is a forge-xyz.jar and a minecraft_server-xyz.jar. When we start mincraft-server the mods don't work properly. The support under Excessively Asked Questions (EAQ) states the following: "The generated server installation operates like a normal Minecraft server, except that you must run the forge-xxx.jar file instead of minecraft_server.xxx.jar." When I start the forge...jar with "java -jar forge-xyz.jar I get the following error message:

A problem occurred running the Server launcher.java.lang.reflect.InvocationTargetException at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.base/java.lang.reflect.Method.invoke(Method.java:568) at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.run(ServerLaunchWrapper.java:70) at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.main(ServerLaunchWrapper.java:34) Caused by: 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.minecraft.launchwrapper.Launch.<init>(Launch.java:34) at net.minecraft.launchwrapper.Launch.main(Launch.java:28) ... 6 more

What are we doing wrong ?