I'm running the latest PROD release of Big Sur and using an iPhone 8 that was on iOS 14.3. Yesterday, I went to restore the iOS software on my iPhone via my Mac, using an official IPSW file from Apple. To install, I open my iPhone in Finder and hold down the Option key while clicking the Update button (which allows me to select the IPSW for installation). My computer processed this file for a few minutes but eventually failed. It seemed like it failed during the package verification step. I found that by going into System Preferences -> Network and then removing the "ESET Network Connection Proxy" and clicking "Apply" I was then able to install the update. I received a pop-up on my machine asking if I wanted to allow or deny the proxy to be reinstalled. I waited until after the iOS update was installed to click "Allow", which restored everything to the way it was originally. iOS 14.4 was released last night so I went to do the same thing this morning with a new iOS 14.4 IPSW and I ran into the same issue. I was able to fix this the same way as I did yesterday (by disabling the ESET proxy), so it seems that ESET is interfering with the update somehow (perhaps a pinned SSL cert is failing a check due to proxying system traffic through ESET). As this solution is not obvious for the average end-user, I'd recommend implementing some kind of bypass for potentially disrupted system services, or at least creating a KB article detailing this workaround.