I still have this problem on my Dell XPS M1330 with and without linux-
backports-modules. For my case, removing iwlagn and re-add it solved the
problem. I checked /var/log/wpa_supplicant.log and found

No configuration found for current AP.

When WPA2 joining is failed. When remove and re-add driver, this line
never appeared again on the log.

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[iwlagn] networkmanager applet cannot connect to WPA2 home network (intel 4965)
https://bugs.launchpad.net/bugs/348275
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