Sometimes I am stuck at this position when using web updater:

And Retry update would fail, because the reason is that the updater secret is meanwhile outdated:

That is the same secret with which the previous "step 4" (download) succeeded. The previous backup step took 15.3s.

Something must have reset or deleted the updater secret in this process.

Opening the admin overview page and from there opening the updater again, let's me finish the update.