It worked but then couldn’t activate the “premium” or whatever it suggested - I don’t remember the exact wording.
It asked if my Sunrise package included TV XL or something, which doesn’t map onto whatever UPC calls it. However, just running the app directly from the UPC TV box sorted that out.
tldr: the UPC login works to log in, but in my case it couldn’t authenticate which UPC TV package I had. I don’t know if my UPC package gives me “premium”, but whatever happened, the message went away 🙂