Binding an OptionGroup to List

Hello Guys,
We use an OptionGroup to let a User select one or more Items. The OptionGroup has a Binder to a HashSet. Now every time we try to commit, we get the following Exceptions: 
com.vaadin.data.fieldgroup.FieldGroup$CommitException: Commit failed
com.vaadin.data.util.converter.Converter$ConversionException: Could not convert value to Set

We already tried to write a Converter for it, but this doesnt work either:

public class RollenConverter implements Converter<Object, Set<Rolle>> {
    
     private BeanContainer<Integer, Rolle> container;
     private final static Set<Rolle> MODEL_TYPE_INSTANCE = new HashSet<Rolle>();
     private OptionGroup opg;
    
     public RollenConverter(AbstractSelect field) {
        this.container = (BeanContainer<Integer, Rolle>) field.getContainerDataSource();
        opg=(OptionGroup) field;
     }
    
     @Override
     public Set<Rolle> convertToModel(Object value, Class<? extends Set<Rolle>> targetType, Locale locale)
            throws com.vaadin.data.util.converter.Converter.ConversionException {
        
        Set<Rolle> models = new HashSet<Rolle>();

        if (container.size() > 0) {
            for (Object itemId : ((HashSet<Object>) value)) {
                if (container.containsId(itemId)) {
                    models.add(container.getItem(itemId).getBean());
                }
            }
        }
        return models;
        
     }

     @Override
     public Object convertToPresentation(Set<Rolle> value, Class<? extends Object> targetType, Locale locale)
            throws com.vaadin.data.util.converter.Converter.ConversionException {
        
        for(Rolle r:value){
            if(opg.containsId(r.getId())){
                opg.select(r.getId());
            }
        }
        
        return value;
    }

    @Override
    public Class<Set<Rolle>> getModelType() {
         return (Class<Set<Rolle>>) MODEL_TYPE_INSTANCE.getClass();
    }

    @Override
    public Class<Object> getPresentationType() {
        return Object.class;
    }

}

Has someone an idea whats wrong? Please help