Cause:
The error is caused by BeanType.writeSchema(Element) method. If the type is an extention, a 'complextContent' element is created which then become the 'root' element for the rest of this method.
The 'abstract' attribute is added to this root element after it has been replaced by the 'complexContent' element. Therefore this error only occurs if the abstract class extends another (abstract) class itself.

Fix:
The fix is easy. The small codeblock to add the 'abstract' attribute is put before the codeblock which creates the 'complexContent' element.
I attached the complete code of the BeanType class including this fix.
Code snippit :

/*

• WSDL error fix. Adding abstract attribute before creating complexContent element
• See Java Virtual Machine specification:
• http://java.sun.com/docs/books/vmspec/2nd-edition/html/ClassFile.doc.html#75734
  */
  if (((info.getTypeClass().getModifiers() & Modifier.ABSTRACT) != 0) &&
  !info.getTypeClass().isInterface()) { complex.setAttribute(new Attribute("abstract", "true")); }

/**

• WSDL error fix : create complexContent after abstract attribute is added to complexType element
  */
  if (info.isExtension() && sooperType != null) { Element complexContent = new Element("complexContent", SoapConstants.XSD_PREFIX, SoapConstants.XSD); complex.addContent(complexContent); complex = complexContent; }

I attached the BeanType source including this fix.