أنشطة "hein.pauwelyn@savaco.com"

Did you also change the _configuration.GetConnectionString("Default") to _configuration.GetConnectionString("<user-secret-key>")?

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
    { 
        if (!optionsBuilder.IsConfigured)  
            optionsBuilder.UseSqlServer(_configuration.GetConnectionString("<user-secret-key>")); 
    } 

Both keys are the same. See the code from the question.

Breakpoints inside OnConfiguring will not be hit.

Hi, I guess you are storing your connection string as a user secret. Can you replace the Default keyword with your user secret key on your ConnectionStringName attribute?

It's defined top of your DbContext class:

[ConnectionStringName("<user-secret-key>")] 
public class MyProjectNameDbContext {} 

By adding [ConnectionStringName("Default")] to the DbContext I can see that the break point inside the ConfigureAzureKeyVault method is hit but the error is not solved. Same error.