We looked into this issue in a connect meeting... Oddly the page /webassist/kfm/get.php would return a 403 or 404 error when passed a url parameter like: /webassist/kfm/get.php?id=1
But we found that creating a copy of the page with a different name would be accessible.
So we did that and did a find and replace on the whole site with get.php and the new page name and that corrected the problem.