My colleague encountered the same problem. He tried to restart with safe mode, but failed. So he tried resetting NVRAM by the following steps： Shut down his Mac  Hold down Option + Command + P + R for about 20 seconds Restart his iMac. Finally, the upgraded was finished. Hopes it helps. checkout this article: setapp.com/how-to/fix-macos-big-sur-issues .

Still facing this issue with iOS 14.5.1. I use a self-signed cert for my apache server to build an OTA. It works just fine until my phone was upgraded to iOS 14.5.1. Nothing helped, even if I've done the following steps: Checked https in manifest.plist. Trusted self-signed cert in Settings - General. Upgrade Apache version, openssl version and update self-signed cert to support TLS1.3. Any suggestion would be appreciated!