The details you posted confirm that your server is capable of talking to PayPal's server and getting a valid response back.

I'm assuming that you posted here because you were *also* getting "Error 52" emails?
Since I've never received an "Error 52" message in any of my testing, development, or live use, I can only assume that the cause is something broken in the files on your server.

You have stated that you're using a very old version of Zen Cart, which also has known security vulnerabilities. If you haven't patched that version using the patches published, then maybe you've been hacked. Even better, you should upgrade to get the benefit of hundreds of additional bugfixes, several of which are related to PayPal as well.