Have you check the dotnetnuke.dll Version on the server, as Sebastian mentored?  A re-run of the SQL install scipt is not possible.  You have to go back from backup for the database and files, then re-run the upgrade.

It could also be a missmatch of the maschineKey in the web.config file.

So best way is go back from backup and starting backup from scatch.  If you not have any backup it would be to hard to fix it. 

No luck on the machine key.

So I guess the "official" word from DNN is "you're up the creek, thanks for using DotNetNuke". I'm hoping the next update miraculously fixes the issue when it runs its install scripts.

After years of using this platform, I think I'll be looking into other options. Though I don't like it nearly as much, at least Wordpress doesn't seem to completely destroy itself if its upgrade script doesn't complete.

Backup made, backup restore tested as I have always done. Something happened and the backup is no longer restorable. Sometime between the test and the upgrade, the backup got corrupted. So now I'm stuck with this problem. 

And no, every CMS I have ever used in the past let's you run the upgrade again and it is smart enough not to try to reapply changes that were successfully made. The fact that the main solution is "just restore a backup" rather than "try to re-install again" is a design flaw.