Hi Absolute,
I've tested PayPal payments with debugging both on and off and I get no info back to Zen Cart in either case. If this wasn't what you meant, I'd appreciate any clarification.
PayPal was working fine on this site on version 1.3.7, I upgraded to 1.3.8, and now no info goes back and forth between PayPal and Zen Cart. Yes, I did remove the module and reinstall it after upgrading.
The only change I see in the admin between the two versions is the sudden inclusion of url_fopen as being a requirement, so that's what I'm considering as the prime suspect, but it's certainly possible I'm having another problem. I just don't know how to test it out at this point.
I do have the 1.3.8 site back up on a test server, though, and would love to try making any changes anyone thinks may help. I'm out of ideas! Thanks in advance.