In the jenkins-1.622 log4j was suddenly replaced by log4j-over-slf4j without any warning or even mention in the release notes.

log4j-over-slf4j jar has only partial implementation of log4j classes and interfaces. Many are absent or have an empty implementation.
That breaks plugins that use log4j.

I got the following errors:

java.lang.NoSuchMethodError: org.apache.log4j.WriterAppender.<init>(Lorg/apache/log4j/Layout;Ljava/io/OutputStream;)V

java.lang.NoSuchMethodError: org.apache.log4j.Logger.removeAppender(Lorg/apache/log4j/Appender;)V

It is possible to work around by using <pluginFirstClassLoader>true</pluginFirstClassLoader> , but that solution can potentially lead to classloading issues.