This error just means the webpage could not be loaded successfully. It could be for any reason, unfortunately… But given that customize.py seems to work fine, you could tell that this has to do with matrixdef, most likely, e.g. if you had any invalid syntax in it (such as beginIter x ending with endIter y).
Your best bet would be doing binary elimination of your changes. Would it be possible to revert to an earlier revision which did not cause the error? Did you keep track of the last satisfactorily working revision, perhaps? After that, compare the versions of matrixdef and add the diffs back gradually (as described here: http://www.codelord.net/2012/04/10/using-binary-search-for-debugging/).
It could also be a permission issue, though if you weren’t having it before, it is unlikely.