Read
http://docs.jboss.org/drools/release/latest/optaplanner-docs/html_single/index.html#logging

"error: Log errors, except those that are thrown to the calling code as a RuntimeException.
Note

If an error happens, Planner normally fails fast: it throws a subclass of RuntimeException with a detailed message to the calling code. It does not log it as an error itself to avoid duplicate log messages. Except if the calling code explicitly catches and eats that RuntimeException, a Thread's default ExceptionHandler will log it as an error anyway. Meanwhile, the code is disrupted from doing further harm or obfuscating the error."