Service Portal Page designer not working while clicking on preview, it takes back to homepage.
The issue is happening because of the fact that the default service portal SP is deleted in the instance or the URL suffix has been changed.
Preview option always uses sp to display the page. As the /sp is not available on the instance, it is getting redirected to the nav page based on the redirection script.
Kindly follows the below process to get the preview page.
1) Click on page designer.
2) Click on sp on the designer header
3) Select any existing portal from the list
4) Then click on preview.