The certificate chain did not validate, no local certificate found

Common example errors:

Unable to authorize payment: The certificate chain did not validate, no local certificate found, Cert Path = c:\pfpcerts\, Working Directory = c:\windows\system32\inetsrv

or

Result Code: -31
Response Message: The certificate chain did not validate, no local certificate found, Cert Path = c:\pfprocerts, Working Directory = c:\windows\system32\inetsrv
Transaction ID:
Auth. Code:

or

I’m Sorry, there was a problem with your transaction.

Transaction Result:
The certificate chain did not validate, no local certificate found, Cert Path = certs, Working directory = c:\winnt\system32\inetsrv

Well, as of January 4, 2010, your Payflow Pro SDK expires permanently and your payment processing will stop if you are using an outdated SDK. You can see this on their website here:
https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/gateway_update_FAQ

Depending on the method or scripting language you are using this can be an easy or difficult thing to get around. As I am Miva Merchant savvy, If you are using Miva Merchant 5, run your updates. If you are running Miva Merchant 4, you can get an updated version of the Payment Module and instructions for instillation at https://support.mivamerchant.com/supportsuite/index.php?_m=downloads&_a=viewdownload&downloaditemid=46&nav=0,12. Zen Cart forums make reference to this change here http://www.zen-cart.com/forum/showthread.php?p=823250#post823250. Anyway, if you have a hand coded cart, visit the top link I supplied for more information. If you find specific articles or information for specific carts, please comment. I will gladly update this post.