Error logs - Best place to look for "Global Error Receipt"


I'm troubleshooting an existing website that has a Moneris payment form. The payment form has been live and functional for some amount of time (months, years?), successfully processing client payments. I'm being told some (all?) users are receiving error code "Global Error Receipt", however I have no further details.

QUESTION: Where's the first place to look for error entries re: "Global Error Receipt" ? Anything on Moneris' side?

The website is Drupal-based, with a custom module that includes the Moneris eCommerce Unified PHP API library, version 1.0.14.

The custom PHP doesn't have explicit statements that specifically log Moneris responses.

Could the errors be logged elsewhere on the server? I would guess no.

I have a mirror of the payment form configured using Moneris test mode and test credentials. I'm able to get successful transactions using dummy CC information.