Masked Download Link broken

I am getting this error when I click on the masked link:
Warning: strstr() [function.strstr]: Empty delimiter in /home/suppeoc/public_html/_suppe/wp-content/plugins/membership/membershipincludes/classes/membershippublic.php on line 422
404 — File not found.

Could someone please help me to fix this. I am trying to get this download nightmare working for 10 hours by now.

Thanks and cheers, Alex