SMTP Error: Could not authenticate

Hi FluentSMTP support team,

I have installed FluentSMTP in one of my websites. It works very well.
Now I want to install it in another one, but it doesn’t work!

I get this error message: “SMTP Error: Could not authenticate.”

Yet I followed the same procedure and respected the same settings.
For both, I used the wp-config.php file to store credentials.
What did I miss for this last installation?

Thanks,