I couldn't yet figure out where but at some place there must be some sort of debug logging left, I see a couple of

com.gargoylesoftware.htmlunit.DefaultCssErrorHandler@54478411

in my console output. Looks like some System.out.println(someObject).

HtmlUnit 2.43.0 - hmtlunit-driver 2.43.1.

/edit: even with SilentCssErrorHandler in place