Yeah actually i see that too on the feedback button... broken buttons at the bottom on all our PDL clients.
Did you actually log the issue with support or did they add the devices and close your ticket?
Support also could add my devices, but as they where adding to Test/DEV org for a customer I left the ticket open hoping it hits the radar of someone able to fix the webpage issue going forward.
On a side note another engineer on my team is going to try to add the SNs via API to a PDL customer.