I can’t add that one either, but I could add another one. Sometimes youtube are added but are empty and without name.
Newsblur seem to be doing a bunch of weird things with youtube. Most of the time they get added as gdata.youtube.com even though that isn’t the official feed URL and doesn’t actually work if visited in a browser.
During the outage I tried using openrss which should work like openrss.org/youtube.com/@britishfilminstitute, almost want to use them anyway since the filter shorts and upcoming videos, however newsblur seems to bypass them add try to add the regular newsblur feed anyway.