As far as I have experienced, in action.default.php, line 42 (starting with
$ext=...) the assumption is that $feed has a '.'.

When $feed does not have a '.', $ext becomes equal to $feed and $feed becomes
equal to '' and the following db query fails resulting in a blank page with the
stat comment i.e. <!-- 0.8756 / 17 / 14225296 / 14884256 --> and no feed.

$feed does not have a '.' when internal_pretty_urls and assume_mod_rewrite are
off.

A possible working solution would be to replace line 42 with:
$ext = strrchr($feed,'.');

Thanks for the module!