Sometimes IIS will present the wrong SSL certificate. IIS can either show the old certificate before you updated it or present an SSL certificate for the wrong site.
First thing to do is to check the bindings, to make sure the site’s got the right certificate.
Next, if you’re using a proxy server, you may need to update the certificate there.
Finally, there is a bit of a bug in IIS where it may present the wrong certificate if you have multiple sites on the same IP / port and are separating these out using SNI. To fix this, remove all the bindings from the server and add them backone by one (restarting IIS after each then checking to see if it’s working). This will help you identify the buggy combination of IP / ports / certificates. Often doing this will fix the issue.