What factors may cause System update import to fail?

from ZCM2017.x to 2020.x even 23.4, on testing lab or production I saw some times that import system update then show error like below

it maybe single or both.

I usually delete this update and them re-import it again. it maybe fine or still fail (I will use take another action: VM snapshot. like today)

But I'm curious why I may get different answers when importing in the same System update, or when re-importing or rewinding the snapshot and importing a new one (but it's also possible that it remains in a failed state)
In the failed state, do I have the opportunity to manually install the Script or MSI file in which folder of the System update to repair the update service so that it can operate normally?

(I worked on this update service problem all day today (I skipped lunch), but later I encountered the problem of Rebuild Package getting stuck.)

Thanks!!

Wencheng