I unfortunately can't tell you what exactly happened, but it looks like there was an issue with the update (obviously), and ESXi reverted to a previously existing installation on the secondary bootbank, i.e. the version/build from which you updated before.
I've been working with HPE hardware for many years, and here's how I usually update hosts if Update Manager isn't an option, or in small environments, where the hosts do not require additional drivers (ones which are not supplied with the vendor's image).
Make sure that you download the correct Offline Bundle (.zip file), which is Gen9Plus in your case, and upload it to a folder on a datastore.
Find out the profile name required for the update command:
esxcli software sources profile list -d /vmfs/volumes/datastore/folder/offline-bundle.zip
Run the upgrade/update (I recommend that you place the host into Maintenance Mode for the upgrade/update):
esxcli software profile install -d /vmfs/volumes/datastore/folder/offline-bundle.zip -p <profilename> --ok-to-remove --dry-run
Note: "--dry-run" can be used to evaluate what's going to happen without actually changing anything. To run the update, run the command without "--dry-run"
Once the update is done (this may take about 20 sec on a SSD, or up to ~2 min on a SD card), verify that the command returned with s.th. like "The update completed successfully, but the system needs to be rebooted". If that's the case, reboot the host by typing "reboot" in the command line.
If the host doesn't automatically connect to vCenter after the update (this may take a minute or so, after it has rebooted), then right click the host, and select Reconnect, which should then work without the need to enter credentials.