I suspect the problem is in your custom files linked to by the buttons in that HTML block (e.g. “WebsiteIntro.php”). Clicking on the buttons fails to open them for the same reason.
See upgrade instructions, particularly this paragraph:
But if you have ANY custom files on your system it is likely they will contain the “WT_” prefix, and they must all, therefore, be changed to use “KT_”. If you miss one case, either that page or your site will fail to open, and you will have somewhere an error message like “HTTP ERROR 403 ACCESS FORBIDDEN”. Typical custom files would include custom theme files (mystyle.css, myheader.php, mytheme.php, myfooter.php); custom language files, custom “histo” files, custom modules or copied modules, non-kiwitrees html or php files.
(additional red highlighting is the specific issue you have).