If an existing SQL Server database that contains a geodatabase is restored to a new or existing server, and the original name of the database is not retained from when the geodatabase was created, issues can be encountered when trying to connect to the geodatabase through ArcGIS.
The following error could occur when connecting to the geodatabase from ArcGIS if the geodatabase is at version 10.9.1.2.9 or earlier and the original name of the database is not used:
Failed to connect to the specified DBMS instance. A database connection exception has occurred. FDO error: -21472216072
The cause of such an error may be linked to restoring a database in SQL Server Enterprise Manager/Management Studio under a different name than the original backup or datafile.
Enterprise geodatabases in SQL Server cannot be renamed because the name of the original database is hard-coded in database system tables. When the name of a database has been changed using a stored procedure or a database is restored to a name different from the original, the database cannot be accessed via ArcGIS clients.
ALTER DATABASE MODIFY NAME
If this is not possible, because an existing database shares the same name, the database must be restored to another SQL Server instance for access using its original name.