Tested 2.9.0 RC1 and noticed one issue due to xerces 2.6.2 version being used on one big app. It causes log4j2 failure to initialize (changes are from LOG4J2-1959 ):

Exception in thread "main" java.lang.AbstractMethodError: javax.xml.parsers.DocumentBuilderFactory.setFeature(Ljava/lang/String;Z)V
    at org.apache.logging.log4j.core.config.xml.XmlConfiguration.disableDtdProcessing(XmlConfiguration.java:205)
    at org.apache.logging.log4j.core.config.xml.XmlConfiguration.newDocumentBuilder(XmlConfiguration.java:194)
    at org.apache.logging.log4j.core.config.xml.XmlConfiguration.<init>(XmlConfiguration.java:92)
    at org.apache.logging.log4j.core.config.xml.XmlConfigurationFactory.getConfiguration(XmlConfigurationFactory.java:46)
    at org.apache.logging.log4j.core.config.ConfigurationFactory.getConfiguration(ConfigurationFactory.java:239)
    at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:369)
    at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:237)
    at org.apache.logging.log4j.core.config.Configurator.initialize(Configurator.java:158)
    at org.apache.logging.log4j.core.config.Configurator.initialize(Configurator.java:131)
    at org.apache.logging.log4j.core.config.Configurator.initialize(Configurator.java:101)
    at org.apache.logging.log4j.core.config.Configurator.initialize(Configurator.java:188)