As I said in comment #31 after I upgraded iwlwifi drivers to version 1.1.20 (even though with that not-so-smart way) I haven't experienced the described problem any more.
However, since ubuntu packages ship with an older version of the drivers, users not manually upgrading may still experience this problem. An update of ubuntu packages would be really appreciated.
Matt, thank you for commenting!
As I said in comment #31 after I upgraded iwlwifi drivers to version 1.1.20 (even though with that not-so-smart way) I haven't experienced the described problem any more.
However, since ubuntu packages ship with an older version of the drivers, users not manually upgrading may still experience this problem. An update of ubuntu packages would be really appreciated.