Offline Credit Cards - Some work and others are jibberish

Basically looking for some ideas on what might be causing this issue.

I have a client that is using an older offline credit card payment solution with OC 1.5.6. I'm not sure which payment extension it is. I can't seem to find an existing match in the Opencart marketplace... so maybe this is an extension that is no longer available. It's simply named 'Offline Credit Card Payment'.

The issue they are having is that some credit card numbers come through fine and display correctly in the admin and others appear as jibberish. It's very inconsistent. On the same day, many orders are fine and many orders have the credit card numbers appear as if the character encoding is messed up.

I know this can sometimes happen with a different encryption method but they are all using the same encryption method.

Any ideas?

Thanks in advance!

It difficult to know without knowing how the module actually works.

You could start with looking at the User Agent: and Accept Language: fields for correct and incorrect orders. The something in common for the ones that don't work.