Symptoms

There is a known issue, that after upgrading FC12 R3 to the FC12 R4 in case of usage SQLExpress version older than 2016 SP2, the application server cannot use previously created DB any more.

Resolution

To solve the issue, SQLExpress has to be updated to the supported version.
In order to save the databases that were previously in use, please follow the next instruction:
Before the update of the SQLExpress version, launch Microsoft SQL Server Management Studio and do the DB backup. To do that, right-click on the DB you need to back up, navigate to the ‘Tasks’ menu and then to the ‘Back Up’ option.

On the opened window, check the location for the backup and press ‘OK’.

Once you have a copy for each of your DB, you can delete the old version of SQLExpress.

After the previous version would be cleared, the new one may be installed. After the installation will be complete, launch the Microsoft SQL Server Management Studio again.

Right-click on the ‘Databases’ and click the ‘Restore’ option.

In the opened window, select ‘Device’ as a source and specify the path to your DB’s backup.

In the left navigation menu, click Options. In the pane on the right, select Overwrite the existing database (WITH REPLACE) and Close existing connections to the destination database (if available).

Then click ‘OK’ and wait for your DB to be restored.
Now DB can be used with the FC12 R4 build.