Credit card payment no longer accepted

Registrations being submitted for Credit Card have started receiving the error
“Registration could not be created:” with no further message or errors.

According to the description in the Customize Feedback Messages section in the plugin settings, this error is shown “When a booking cannot be made due to an error when filling the form. Below this, there will be a dynamic list of errors.”

However the form contents are correct and if we change payment method to “Pay Offline” the registration is accepted (so the form contents must be acceptable).

We’ve spoken to our payment gateway provider (Authorize.net) and they have advised that they changed their SSL certificate yesterday. Is it possible that the old certificate is being used for validation by the plugin somehow? They provided this link to the new certificate:

Entrust Certification Authority – L1K
AIA: https://aia.entrust.net/l1k-chain256.cer

If it’s not a payment gateway problem (and the error message seems to appear very quickly, maybe too quickly for an external certificate to be checked) what else should we be checking?
We are running the latest plug in version at this time (5.9.2)

The page I need help with: [log in to see the link]