Comment 14 for bug 1573720

network-manager just got updated to 1.2.2 containing a fix for this bug:

https://bugs.launchpad.net/ubuntu/+source/network-manager-applet/+bug/1575614

I assume that this issue is actually a duplicate of 1575614.

As it seems it required two steps for me to fix this problem:

* Update BIOS (here: Dell Precision 5510 updated from 1.2.10 to 1.2.14)

Having this I already was able to connect to the protected WLAN... but had to ignore some nag-screens about the "unencrypted" key.

* Update network-manager to 1.2.2 (available through system update)

Private keys are now visible in the file-chooser and I get no more complaints on the "unencrypted" key.