groovy compiler problem ("is not abstract and does not override setMetaData...")

hi,

I got a problem compiling a groovy test class (using webtest).

Following compiles:
===
class BlaTest extends GroovyTestCase{
  void testBla() {   }
}


Following does NOT compile:
===
import com.canoo.webtest.WebtestCase
class BlaTest extends WebtestCase {
  
  void testBla() {  }
}


The error message is: "BlaTest is not abstract and does not override abstract method setMetaClass() ....."
The webtest library is  included as a maven module (normal maven run is executing the test-class without problem).
I am using   9.0.1

Are there any ideas what could be the cause?

thanks