Did Vatican II's documents actually contain errors, or did liberals just misinterpret them?

I do not think they hold the modernist interpretations to be correct; that would not make sense. The real issue divides into two. One camp in the SSPX holds that certain paragraphs of a few Vatican II documents are simply wrong, that the Fathers got them wrong, in things like Dignitatis Humanae, Dei Verbum, Unitatis Redintegratio, and Lumen Gentium, and that the liberals wrote them so as to interpret them their way afterward. The other camp holds that the Vatican II Fathers did not get anything wrong, though they will not call the council infallible, and that it was the liberals coming out of the council who distorted and misinterpreted the documents. I am one who is convinced that Vatican II did not get it wrong; it was the liberals, who were thoroughly in and out of the council trying to move it their way, and were sometimes almost successful until Paul VI stopped them, as when he stopped the errors on the inspiration of Scripture in Dei Verbum. I have a whole book on that, The Demise of Catholic Scripture Scholarship. Those differences all have to be ironed out.