Thank you so much for your quick reply Till,
I created a mail domain (with the same name) via ISPconfig web interface.
Then I restored only the records of this particular domain to the mail_user table
And then, I restored the /var/vmail/(domain) directory.
Now, everything "looks" right in the ispconfig web, but users cannot login to their mail account.
Jul 2 19:18:51 h12 dovecot: imap-login: Aborted login (auth failed, 1 attempts): user=<firstname.lastname@example.org>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured
If I create a new email account in this domain, it works fine.
What I am missed?