I had a similiar problem and ended up tracing the error to the includes/functions/cache.php and the reference to the old categories.php file which needed to be corrected to the new dm_categories.php file:
////
//! Cache the categories box
// Cache the categories box
function tep_cache_categories_box($auto_expire = false, $refresh = false) {
global $cPath, $language, $languages_id, $tree, $cPath_array, $categories_string, $boxContent, $box_base_name;
if (($refresh == true) || !read_cache($cache_output, 'categories_box-' . str_replace ( '/', '-', bts_select('boxes', $box_base_name)) . '-' . $language . '.cache' . $cPath, $auto_expire)) {
ob_start();
include(bts_select(boxes_original, [b]'dm_categories.php'[/b]));
$cache_output = ob_get_contents();
ob_end_clean();
write_cache($cache_output, 'categories_box-' . str_replace ( '/', '-', bts_select('boxes', $box_base_name)) . '-' . $language . '.cache' . $cPath);
}
return $cache_output;
}