I just want to chime in a say I've had this issue with paypal since the beginning may as well and my host says everything is "fine" on their side and that I should check with the zen cart developers. I'm pretty frustrated (and mostly venting my frustration ) I've run the ipncheck tool and the prod api seems to be ok but get errors on the sandbox check. tried the ipncheck today and its not working. go figure. Is there something else I should check? switch to a different payment system, maybe different host?