Minecraft 1.12.2 modded server: "problem occured" - minecraft

Question

I have a minecraft modded server on 1.12.2, and it works for the most part, but it gives me an error.
The minecraft server is
forge-1.12.2-14.23.5.2846-universal.jar, and I'm on lubuntu, using openJDK 10.
I have these mods:
GalacticraftCore-4.0.2.236, GalacticraftPlanets-4.0.2.236, MicdoodleCore-4.0.2.236, and hopperducts 1.5
My system is:
Distributor ID: Ubuntu
Description: Ubuntu 19.04
Release: 19.04
I tried reinstalling it, and it worked to get mission libraries(some) and then this came up.
My code to run is:
java -Xmx1G -Xms1G -jar forge-1.12.2-14.23.5.2846-universal.jar
where "forge-1.12.2-14.23.5.2846-universal.jar" is the run file(yes, spelled correctly).
I just expected it to run, but it gave me this error:
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:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
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

Answer 1

For posterity's sake, the issue is openjdk-10. The server .jar file uses something or other that isn't supported by versions of java after 8, and throws this same error you saw, that I spent a while chasing down. There's another relevant thread here. Solution: remove openjdk-10 (or whatever version you're using that isn't 8) with
sudo apt-get remove openjdk-10
and make sure version 8 is installed with
sudo apt-get install openjdk-8.

Answer 2

I found this on GitHub. Basically, the issue may be something with the vannilla Minecraft server.jar. Try re-downloading it, and manually renaming it. If that doesn't fix, there was a comment in there about Linux. Check that out.
Sorry I'm not more help. I'm not a Java developer, and I have just barely used Linux. I used CentOS a bit on my Raspberry Pi, and I used Ubuntu in a virtual machine, but besides that I'm a Windows user.