After upgrading from 12.2 to 12.3, the server application and the database update tool may fail, and return HTTP error 50x or other exceptions.
The issue can be caused by a missing DLL file, which gets removed during the upgrade process. This can be confirmed by going to the Event Viewer, and observing errors reported by .NET app runtime.
To restore the missing dependency file, repair the installation using the original MSI setup for version 12.3. This can be achieved in various ways, for example by right-clicking the .MSI file and selecting the Repair option from the context menu. Windows Installer will check the integrity and restore missing files if necessary.
The problem does not affect clean installations. This means that uninstallation of a previously installed version 12.2, followed by the installation of version 12.3 would be also sufficient to work-around the issue.