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thanks..Welcome to the forums, sergiufcd.
It is likely that an app, widget or service is waking up your screen.
I would go into each cofigurable widget and app and check for auto updating and syncing. I'd also check in the running services area of the application manager, where you can turn off services by clicking.
)I have the same issue, the screen goes dark, after 30 seconds it should lock, but it doesn't.
Other problem is the alarm clock won't start when it should. For example I set it to ring at 7.30. After I see that it passes 7.30 and no alarm, I push a button and it starts to ring. It's like it has amnesia)
Any ideas?

The screen goes dark after 30 seconds and it should be lock,but I would like to immediately lock after call end. How should I do it?