Paypal IPN failing?

Hello community!

I've been using Paypal IPN for several years, and earlier this year it stopped working entirely. I investigated, and the Paypal IPN notifications are not reaching the script (it is never triggered); instead, the Linode server is returning an HTTP 500 error code when Paypal tries to connect. However, when using the Paypal sandbox to do a test IPN with the same Linode URL and same script, everything works perfectly.

So, I'm pretty sure it's not a problem in my script; rather it is a server or firewall configuration issue, blocking the Paypal production server but not the sandbox server.

Any ideas on how to investigate this further? I looked at the Linode firewall rules on the account, and there's nothing there. I've tried everything I can think of, and really hitting a block here. Looking forward to your thoughts, thank you in advance!