When testing notifications always remember that if you push a file through a transition, you will not get notified. For example if you setup a notification for the design group which you are a member of and you push a file through, everyone else in that group will get notified but you won't. Other than that if you are using SMTP notifications then try switching to database notifications and see if that works. Also if you made Vault A from a database backup of Vault B (common when settign up a test environment) make sure you followed the procedure correctly as that can cause this issue as well. Or if you followed the procedure to rename your vault make sure eveything got set correctly.
I already tried as if it's in a real environment (do transition as users with the assigned rights and permissions respectively each)
And yes, I'm using the database notifications.
Unfortunately, it still does not work. I tried to create a new vault of this, still no notification available.
Once go to SQL n look for Entries in DocumentActionInfo table.Did you rename this vault or Something?
I suspect a problem with Database Server.