How is the admin app configured in the IIS? From the URL format it seems that the app is nested underneath the live site app. This is not supported. The apps must be running side-by-side!
Although this should not be the cause of the issue, it can cause issues with page builder and others. I would recommend to host the admin app in a separate site and then just use the /admin redirect on the live site to the admin site.
The error you are getting - I would recommend restoring backups and following the hotfix instructions closely, as it looks like the hotfix was not applied correctly. Including the steps after the hotfix was applied and additional notes.
It seems that some data in the DB were not updated.