When not specifying Groovy dependency and starting application with jettyRun, logback will report unambiguous "GroovyCastException". However, "Groovy classes are not available on the class path." is expected. More info:

http://stackoverflow.com/questions/10995414/unable-to-use-logback-groovy-but-logback-xml-works/

I attached a simple project that demonstrates this problem. It requires Gradle installed. To run it, in console type "gradle jettyRun" from the project folder.