I have solved my issue doing the following:
1. active roundcube logs:
At the end of /var/www/roundcube/config/config.inc.php, add:
// LOGGING
$config['log_driver'] = 'syslog';
$config['syslog_facility'] = LOG_MAIL;
2. restart apache2
sudo systemctl restart apache2
3. Reconnect to Roundcube, Settings - Identities, click on the username and generate the error.
4. Check roundcube errors, in my case they were in /var/log/syslog and /var/log/mail.err
I found the following errors:
Jan 14 18:03:30 mail roundcube: <n6rvhgna> PHP Error: Can use only one plugin for attachments/file uploads! Using 'filesystem_attachments', ignoring others. in /var/www/roundcube/plugins/filesystem_attachments/filesystem_attachments.php on line 39 (GET /?_task=settings&_action=edit-identity&_iid=13&_framed=1)
Jan 14 18:03:30 mail roundcube: <n6rvhgna> PHP Error: Enigma plugin: Option 'enigma_pgp_homedir' not specified in /var/www/roundcube/plugins/enigma/lib/enigma_engine.php on line 91 (GET /?_task=settings&_action=edit-identity&_iid=13&_framed=1)
From there, I decided to deactivate enigma from config.inc.php, by commenting the plugin out in the file:
// 'enigma',
and restart apache2 again.
Now Settings - Identities allows user identity modifications and saves them as expected.