Contribution form errors after latest security release
So the error occurs when CiviCRM is doing it's implementation of the Luhn algorithm https://en.wikipedia.org/wiki/Luhn_algorithm

The picture suggests it's javascript related, but note that there are no console errors showing.

Fix: I cleared both the site cache and the browser cache and now it works.

The weird [object Object] title that you can see in the console seems to be a red herring.

In our case, the issue is likely due to css/js/img resources having long caches and not getting cleared by CiviCRM with the upgrade, so restarting Varnish in front of the site is likely the right answer.