If you or your clients are viewing the event pages in Internet Explorer 8 using Windows XP (neither are now supported by Microsoft), they may see a warning message saying the SSL certitifcate is not valid.
This is caused because it does not support Server Name Indication (SNI).
There are two options:
1. Upgrade to a modern browser (this is not always possible, especially in large organisations)
2. Contact us and we will use a workaround for the particular event domain (a small charge will be incurred for this).