I installed an SSL certificate on an Apache web server, running on Ubuntu 14.04, approximately 3 months ago. The certificate was purchased through Digicert and is valid through 2017. However, about a week ago the site started displaying an error message, saying the certificate was invalid. I restarted Apache and that seemed to solve the problem. But just yesterday the error returned. I restarted Apache a 2nd time and again, everything is working fine. But now I'm concerned that rebooting is just a temporary solution.