We are getting the error "The page you are looking for could not be found" when trying to open Knowledge Base \ Notification settings

The system property 'glide.knowman.serviceportal.portal_url' is currently set to 'kb' which is the same as out of box:
/nav_to.do?uri=sys_properties.do?sys_id=33cef466db41320002533691f0b8f50b

However, there is currently no portal with this suffix in your instance in the instance. It looks like the OOB Knowledge Service portal suffix has been changed 

In this case, the property is redirecting users to portal '/kb' in your instance, but this does not exist since it was changed. This is why we are seeing the issue. If you change the URL manually, or just go to the following URLs you can see the notification page loads properly:
/sp?id=actsub_notif_pref&sysparm_domain_restore=false&sysparm_stack=no

The solution is to update the system property 'glide.knowman.serviceportal.portal_url'. 

Knowledge Management Service Portal properties
https://docs.servicenow.com/bundle/newyork-servicenow-platform/page/product/knowledge-management/reference/knowledge-service-portal-properties.html