As I already wrote
here, I was able to break down the problem to the encryption of the passwords.
Before storing the SMTP passwords to the database, they are being encrypted here:
$password = rcmail::get_instance()->encrypt($password);
When sending a mail, the passwords are being decrypted here:
$args['smtp_pass'] = rcmail::get_instance()->decrypt($smtpSettings['smtp_pass']);
I disabled both the encryption and decryption, and now the plugin works again, with the downside that the passwords are unencrypted in the database.
Do you have an idea how to fix the password encryption?
Also: sorry for late reply. I must have missed the reply notification.