Error: "Cannot Open User Default Database. Login Failed". (4378903)

The login error occurs when a SQL Server user cannot connect to their default database. The reasons may include: 

• Permission Issues:
  • The login does not have a mapped user in its default database.
  • A user mapping was not created for the login's default database.
  • The mapped user has been denied access, such as being added to the db_denydatareader role.
• Database Availability Issues:
• The default database is in suspect mode.
• The default database does not exist, or the name is incorrect.
• The default database is in single-user mode and is already in use.
• The default database is offline, detached, or in an EMERGENCY state.
• The default database is in RESTRICTED_USER state or part of a database mirror.

Additionally, login accounts belonging to multiple groups may encounter issues if the default database for one of the groups is unavailable.

Contact the MS SQL Server system administrator to resolve the issue. They can:

1. Verify the user's default database using the following query:
   copy

   SELECT name, default_database_name FROM sys.server_principals WHERE type = 'S' AND name = '<sql-login>';
2. Address any permission problems:
   • Ensure the login has a mapped user in the default database.
   • Create the necessary user mapping for the default database.
   • Remove the login's mapped user from restrictive roles, such as db_denydatareader.
3. Resolve database availability issues:
   • Restore the default database if it is missing or unavailable.
   • Change the default database to a valid and accessible one