ch.qos.logback.classic.joran.action.ConfigurationAction.begin(InterpretationContext, String, Attributes) tries to read a system property.
if logback is used in an applet, there's the applet's security manager which raises a SecurtyException.

the full stacktrace is:
11:15:36,658 |-ERROR in ch.qos.logback.core.joran.spi.Interpreter@1:16 - RuntimeException in Action for tag [configuration] java.security.AccessControlException: access denied (java.util.PropertyPermission logback.debug read)
at java.security.AccessControlException: access denied (java.util.PropertyPermission logback.debug read)
at at java.security.AccessControlContext.checkPermission(Unknown Source)
at at java.security.AccessController.checkPermission(Unknown Source)
at at java.lang.SecurityManager.checkPermission(Unknown Source)
at at java.lang.SecurityManager.checkPropertyAccess(Unknown Source)
at at java.lang.System.getProperty(Unknown Source)
at at ch.qos.logback.classic.joran.action.ConfigurationAction.begin(ConfigurationAction.java:42)
at at ch.qos.logback.core.joran.spi.Interpreter.callBeginAction(Interpreter.java:276)
at at ch.qos.logback.core.joran.spi.Interpreter.startElement(Interpreter.java:148)
at at ch.qos.logback.core.joran.spi.Interpreter.startElement(Interpreter.java:130)
at at ch.qos.logback.core.joran.spi.EventPlayer.play(EventPlayer.java:50)
at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:157)
at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:143)
at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:106)
at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:56)
at at ch.qos.logback.classic.util.ContextInitializer.configureByResource(ContextInitializer.java:75)
at at ch.qos.logback.classic.util.ContextInitializer.autoConfig(ContextInitializer.java:148)
at at org.slf4j.impl.StaticLoggerBinder.init(StaticLoggerBinder.java:84)
at at org.slf4j.impl.StaticLoggerBinder.<clinit>(StaticLoggerBinder.java:54)
at at org.slf4j.LoggerFactory.bind(LoggerFactory.java:128)
at at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:107)
at at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:295)
at at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:269)
at at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:281)
...
at at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at at java.awt.EventQueue.access$000(Unknown Source)
at at java.awt.EventQueue$1.run(Unknown Source)
at at java.awt.EventQueue$1.run(Unknown Source)
at at java.security.AccessController.doPrivileged(Native Method)
at at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at at java.awt.EventQueue.dispatchEvent(Unknown Source)
at at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at at java.awt.EventDispatchThread.run(Unknown Source)
11:15:36,661 |-INFO in ch.qos.logback.classic.joran.JoranConfigurator@10bbf9e - Registering current configuration as safe fallback point