One of the issues that can cause this is an intermediate certificate break in your SSL chain.  You can verify the validity of the SSL certificate chain using an online SSL Certificate checking tool.  

If the SSL certificate in question is installed to a NetCommunity web server that is hosted by Blackbaud, Click Chat with Support and reference this article.

If Blackbaud does not host your NetCommunity web server, please reach out to your web server administrator and/or the issuer of the SSL certificate for further assistance.