I got it working now. I have overriden the bind-method of the MultiActionController, catching ServletRequestBindingExceptions and doing nothing with them:
The binderror is part of the Errors object with code TypeMismatch and can be shown on the form.
protected void bind(ServletRequest request, Object command) throws Exception
catch (ServletRequestBindingException e)
Is this the only way to handle this with an MultiActionController?