Same error for me, found alternative working solution (wp 3.8.1). I haave permalinks set to custom format with %postname% . Always on line 135 as suggested by djnforce9 i changed:
$wpfib_baselink = site_url().DS.”?p=”.get_the_ID();
to:
$wpfib_baselink = get_permalink().”?x=0″;
// ?x=0 beacuse it then uses &dir=…
and now it’s working fine. However any downloaded file is corrupted :/
Hope it helps!