Having the same issue as a lot of people apparently, Incredible that it hasn't been fixed yet. Recently factory resetted my phone, and now back up is stuck, won't go any further than some 10 photos or so. Tried force stopping it, clearing cache and storage, resetting phone, reinstalling the app, nothing works. Some people were saying things regarding 0kb files, I deleted the ones I found but this hasn't fixed the issue either. Pointless on paying for Google One if I can't even use the backup.