Error scanning entry META-INF/versions/9/org/apache/logging/log4j/util/StackLocator.class with jetty and log4j 2.9.1?

log4j 2.9 and later are multi-release jars for Java 9.

Your Jetty version don't support that. Either upgrade Jetty to a Java 9 compatible version, or use log4j 2.8.x.

See Jetty issue #1797: JEP 238 - Multi-Release JAR files break bytecode scanning.