"Unable to download updates" error with 404 on specific package version
When trying to download updates (by hitting the "download" button on the "updates" tab), I got the following error:
Unable to download updates: E: https://repo.pureos.net/pureos amber-updates/main arm64 tzdata all 2020e-0+deb10u1 is not (yet) available (404 Not Found ...)
I would guess that this happened because the update that the system thought it should do involved packages that aren't available on the server anymore.
Hitting the unlabeled button on the upper left, to trigger a package list update (and then waiting until that finished, which didn't have any kind of progress bar) did not fix the issue; the update changed, but it now wants to download a libzstd1 package that doesn't exist.
Ideally, the software update application would handle this situation, such as by updating the package list (or at least suggesting doing so) if a package isn't available, and by installing as much as possible if a package isn't available.