When using a custom domain name, you may at some points encounter a warning saying "This certificate is not valid." (Depending on what web browser you use, the message may be worded differently.) This is not an error. It is a warning generated by your browser. This message indicates that the Secure Sockets Layer (SSL) certificate for the site is for a different domain than the one you have entered, and thus your browser will not automatically trust the site. All communications with the site are still encrypted.
This message appears because all Ploud sites have a certificate for the ".ploud.net" domain. If a custom domain (for example, www.dclibrary.us) is used to redirect users to the ploud.net domain, and if SSL encryption is turned on (as indicated by the "https://" in front of your URL), then your browser will warn you that the certificate is for a different domain than you've entered.
The following screenshots are examples from different browsers of the message you will see when navigating to a custom domain under SSL.