Thank you.
I checked roundcube .ini settings, but sort is at default values. Your suggestion (thank you !!!) made me think and I checked the sort settings stored inside the user web interface for those problematic folders.
The problem was just that! I set to 'None' + 'Descending' and messages in that folder appeared in les than 2 seconds.
I noticed that if you click on one column header (like date) this setting is set and stored, and the next time you display that folder, last stored setting are applied.
Is there any way not to automatically store that values and set to default for new sessions?
Roberto
// ----------------------------------
// USER INTERFACE
// ----------------------------------
// default messages sort column. Use empty value for default server's sorting,
// or 'arrival', 'date', 'subject', 'from', 'to', 'fromto', 'size', 'cc'
$config['message_sort_col'] = '';
// default messages sort order
$config['message_sort_order'] = 'DESC';