Hi Zhang and thanks for your answer
Your page says that roundcube logs is by default logging to postfix log, which is /var/log/mail.log for debian.
But badfully when i try to create a new folder in roundcube, mail.log doesn't show anything more
#edit: OK i found out by looking on google that by default roundcube logging is not activated. You have to add this:
$config['debug_level'] = 1;
// Log SQL queries to <log_dir>/sql or to syslog
$config['sql_debug'] = true;
// Log IMAP conversation to <log_dir>/imap or to syslog
$config['imap_debug'] = true;
// Log LDAP conversation to <log_dir>/ldap or to syslog
$config['ldap_debug'] = true;
// Log SMTP conversation to <log_dir>/smtp or to syslog
$config['smtp_debug'] = true;
But even like that I don't see any errors when I try to create the folder.
I tried to create a new mail account on my server and guess what, folder creation works with no problems. So this issue is only linked to my mail account.
#edit2: OK i found out the solution like a big boy
I noticed that i had another related bug: when i clicked on one folder to unsubscribe from it, i had this error showing:
Roundcube log in mail.log (after activating logging in config file) showed only this:
A0003 NO [SERVERBUG] Internal error occurred. Refer to server log for more information. [2016-03-26 22:37:09]
After looking on google I found out that this "SERVERBUG" refers to a dovecot action. So i checked dovecot-sieve.log in /var/log/ and found this error:
So i checked permissions on the folder where my vmail dir was stored, and the group permission was defined to root instead of vmail! Which is very suprising since i never changed this permissions since i installed the server and I was able to create folders before then.
So I just did a chown -R vmail:vmail on my mail folder and everything works now