I don't know your backup/restore process so I can't speak to why you aren't able to revert to the previous version.
But given a working earlier version, I can help troubleshoot the upgrade. From a working copy of your original 2.4.3 install, run this SQL against your gallery database:
SELECT SettingValue FROM gs_AppSetting WHERE SettingName = 'DataSchemaVersion'
The Upgrade Wizard is designed to give you the error you saw only when this SQL returns a version earlier than 2.3.3421, so it shouldn't appear if you are running 2.4.3.
Roger Martin
Creator and Lead Developer of Gallery Server Pro