Guys, I want to post here a big THANKS to zombiehype!
I created a post 1 year ago, complaining about the same problem:
http://forums.dlink.com/index.php?topic=64393.0
That time no solution found, I gave up. In fact I've been struggling with this problem during almost 3 years.
Last week, I decided to buy a new one, thinking that my unit was defective. I bought the same model, DNS-320L, trying to give it a new chance.
It worked for the first times and then the same problem appeared. Then started searching again on internet and found my own topic and the link to this topic.
I saw your solution here, and it REALLY works.
I can't believe it's just that. 3 years of struggle just because clicking on "favorite apps" instead of going on "Applications" menu and selecting "Local Backup".
I start to laugh hard here, mad at the programmers of this interface. Now I'll return the new NAS and happily use the old one.
Just to add, working perfectly on last firmware 1.08b08
Thankyou zombiehype, hope it will save someone's else life!