Following a customer suggestion we have added certificate testing to our https monitor. The updated monitor will check the certificate chain, that the certificate is valid for the website it is being used on and that it has not expired. If any of the checks fail the site being monitored will be determined to have failed and you will be alerted as normal.
You can set which of the tests we should perform via the control panel in the configuration for each individual monitor. To get you started we have automatically enabled this feature on all monitors which pass the certificate tests. If any of the tests failed they have automatically been disabled.
The main reason for failures will be, self signed certificates, using certificates on sites for which they are not valid or using an expired certificate. All of these should throw up warnings in your browser when you visit the site unless you have told your browser to accept the invalid certificate.
You may want to review the settings on each of your https monitors. If any options are not enabled and you are unsure try visiting the site and seeing what your browser says before contacting support for advice.