I solved the issue some days ago downgrading to 6.0-7321 following instructions of "Darrend" of May 23rd at 9:42 pm (page 23) at https://forum.synology.com/enu/viewtopi ... &start=330
Now the nas works right as before.
This morning I saw that synology releases a new update of DSM: 6.0.1-7393-1; perhaps it could be solve the issue. But in my case I don't try it, because I don't want to spend more time if the new update doesn't fix the issue.
Fixed the link for those trying to follow it https://forum.synology.com/enu/viewtopic.php?f=39&t=46450&hilit=downgrade&start=330#p436344