That's fine, Martin, no apologies needed. When we defend a part of another's position, or another's scholarship and faith in general, it is sometimes taken for granted that we share the same views.
I'm going to answer your question two ways. First, I believe Wright has made a cardinal error in that he has broken the rule of tradition. Many will object to biblical views contrary to their tradition, thought, or interpretation as being unbiblical doctrine. For example, when I argued a meaning for "forsaken" other than a separation between the Father and Son on the cross, I had made what was in your view a cardinal error. N.T. Wright certainly does this.
But second, in terms of orthodox Christianity, or even Reformed theology in general, I do not believe there is a cardinal error one can put their finger upon. There are differences in interpretation, but there is no such error. The reason that I disagree with Wright is that I do not think he has evidenced his position well enough to accept. Indeed, Wright even says as much (and states his desire is dialogue and research on the topic).
In truth, I am not confident that most here would entertain the doctrine of Luther were we alive during the Reformation and steeped in Catholic dogma. I am, actually, pretty sure many here would have sought to have Luther executed for his views. When our beliefs are challenged we get defensive - not because our beliefs are biblical but because they are our beliefs.
What I do take from Wright, more than his conclusions, are the questions he asks. I believe there are areas where we seem to present the first century Jew as a sixteenth century Roman Catholic. It is difficult to fathom these similarities as being merely coincidental. So I do see merit in his investigation although I do not necessarily accept his conclusions.
As Wright pointed out: "The greatest honor we can pay the Reformers is not to treat them as infallible - they would be horrified at that - but to do as they did."
I believe the same of C.S. Lewis, Karl Barth, Martin Luther, John Owen, Joel Beeke, and Tim Keller (all of whom I have both read and appreciate but also disagree on some point). We all seem to think that everyone else is wrong on at least one thing. We are all probably right. Read, study, learn...but do so with discernment. Take what is good, leave what is bad. But we need to stop making villains out of saints.