it's like poking all around without access to your server
One more try:
Place rss2html.php in the root of your webserver and the add just one more slash:
HTML Code:
{php}
include("/rss2html.php?XMLFILE=/phpBB3/generate_feed.php%3fcontent%3dposts%26f%255B%255D%3d17%26number_items%3d1%26sid%3d3043e6c2e27b05430edc3ddff7d95c4b&TEMPLATE=sample-template.html&MAXITEMS=1");
{/php}