Same problem. Retried several times. Why they don't retain the download cache so we don't have to re-download the entire 3.25GB from the beginning each time is beyond me. The problem is NOT the internet connection. All the "solutions" I've seen so far on this forum are work-arounds (hacks). Apple needs to fix this ASAP. It took me several attempts and a lot of freeing up of disk space to get Big Sur to install in the first place. Now the thing can't even update itself, despite my MBP having plenty of free space. I don't want to reboot into safe mode, stand on one foot, pat my head and rub my belly. I want an updater that A) works or B) at least provides accurate error messages. If I'm going to pay more for a product, I expect it to work at least as well as the competition.