Stack Trace:
java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @7ba3ee2e
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:357)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:199)
at java.base/java.lang.reflect.Method.setAccessible(Method.java:193)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.generateClass(StructureCompiler.java:299)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:213)
at com.comphenix.protocol.reflect.compiler.BackgroundCompiler$2.call(BackgroundCompiler.java:204)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:831)

This is fixed in the latest dev builds. Find them here: https://ci.dmulloy2.net/job/ProtocolLib/