update-database fails with error 0x80131904 (server not found)

Question

My application has connection string

  <connectionStrings>
    <add name="XXX" connectionString="Data Source=(LocalDb)\v14.0;Initial Catalog=XXX;Integrated Security=SSPI; MultipleActiveResultSets=true" providerName="System.Data.SqlClient" />
  </connectionStrings>

in library XXX.Data. But when command update-database is started in PMC, the error is displayed:

Target database is: 'XXX' (DataSource: .\SQLEXPRESS, Provider: System.Data.SqlClient, Origin: Convention).
System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
   at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling)

It tries connect to .\\SQLEXPRESS instead of (LocalDb)\\v14.0;Initial Catalog=XXX ? Why?

I dont have any other connenction string in any dll files.

Update 1: I tried with other connection string but it failed too:

<connectionStrings>
    <add name="XXX" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;Initial Catalog=XXX;Integrated Security=SSPI; MultipleActiveResultSets=true" providerName="System.Data.SqlClient" />
  </connectionStrings>

Answer 1

1. I had to unload xxx.web project containing EF, but it had not connection string.

2. After step 1, it worked for following command with 2 parameters:

    update-database -ConnectionStringName "XXX" -ProjectName "XXXX.Data"