Almost all websites you visit securely will be using TCP 443 which is the well-known port for HTTPS. However if you need to visit another, less well-known port for HTTPS, for example https://yourdomain.com:8443/ this will cause a UTM error.

To resolve this issue you can configure the below.

Go to  “Web Protection” -> “Filtering Options” -> “Misc”

Under “Allow Target Services” click the small plus button

Fill in the circled fields on the above image, the fields being fairly self explanatory.

Once you are happy with your changes press the save button and this will add your port to the “Allowed Target Services” list.  Then when you browse to https://yourdomain.com:8443/  you should no longer get an error from the Sophos UTM.