Today i'd had a look in the error.log of the roundcube.
There was an error
- [15-Oct-2013 06:59:15 UTC] PHP Warning: finfo_open(): open_basedir restriction in effect. File(/usr/share/misc/magic) is not within the allowed path(s): (/var/www/imscp/gui/:/etc/imscp/:/proc/:/bin/df:/bin/mount:/var/log/rkhunter.log:/var/log/chkrootkit.log:/usr/share/php:/usr/share/pear/) in /var/www/imscp/gui/public/tools/webmail/program/lib/Roundcube/rcube_mime.php on line 727
After adding the folder "/usr/share/file" to the paths the error disappears.
Roundcube isn't modified, so i think the problem has anyone.
Can you check this, too?