I have the exact same problem as another user:
am setting up Cubecart site and cannot get the PayPal IPN payment gateway to work properly.
Specifically, when the user returns to the Cubecart site from PayPal they receive a Cubecart error "Order failed"...however, the order actually processes.
The site is setup for digital downloads only. Upon returning from PayPal to the CubeCart site, the user should receive an "Order completed" message and the digital download link should be mailed to the user.
This IPN issue seems to be a common problem with Cubecart. There are a number of mods on their forums, however, none work fully.
I need someone to fix this ASAP.
Please contact me if you have resolved this specific problem in the past.