For some reason we have had the odd user experience SSL certificate errors, particularly with redirects.
I have looked at this in the past and cannot replicate the errors in my Firefox browser but most users report that they are using Chrome when they see this error. For Chrome it was a case of clearing the browser cookies plus the cache and the error usually goes away. Maybe that would be worth a try?
If not then it may be worth trying again with a different browser and see if that helps.
I know our resident coding guru has done some fantastic work on the site and looked at the redirect issue before to no avail. Seeing as the issue can usually be resolved at the user end by the above suggestions and is otherwise a rarity that's probably the best answer I can currently give you.
Let us know how you get on.
All the best,